I'm not simply copy-pasting Jared Diamond, here. In fact, I cited Rome as an economics problem, just as you are trying to argue, which, I'll point out, is an argument I got from entirely different authors, some of whom are very much in favor of economic theories. (Did you simply see the name Diamond and discount everything else I was saying?) Although I should point out, saying "economics is always the cause of social change" - as seems to be becoming more and more popular - is a little disingenuous an argument, since, if you try hard enough
all social interaction can be called economics, which merely makes it tautological. The argument seems to be that a plague is an economic condition, as is deforestation, as were the Conquistadors. At a certain point, a label is either rendered semantically meaningless, or just a tool of oversimplification to force the same predetermined solution onto every problem.
In any event, what you are describing in the OP is not at all a model of economics, it is,
I propose that we add a randomly-generated 'time limit' to civilizations; as this number counts down, their behaviour can change. Then it hits zero, that civilization will experience 'dark age' period and eventually return to the top of the counter, causing its behavior to revert.
Now, I haven't read Tainter's book, myself, and I'm just going off of a synopsis to get the gist of what his point was, but I'm pretty sure that he's not saying that societies just
arbitrarily go into decline just because a cosmic clock struck 12. The purpose of these sorts of mechanics in the simulation are to create the sense of verisimilitude when players look at the world, and see that a once-great city has gone into decline, and can then go ask a scholar "why"? Maybe the scholar has a right answer or a wrong one, but there should at least be
some reason. Or, the player should be able to walk through that society and see the signs of growth or decay, based upon the social structure of that given society. You, yourself, said that this was supposed to be adding to the flavor of the game, but what flavor is there if these civilizations die out for no reason whatsoever? What determines which ones are abandoned, if this is all random, and if so, what determines which ones get reclaimed?
Without being able to answer these things, you don't have a modeled social theory, you have a label stretched across a gross oversimplification of behavior.
(In fact, reading the synopsis, Tainter suggests two ways in which social degradation can be avoided: Technological advancement and interdependency with other complex societies. Neither of these factor into your initial model.)
I would also point out, in previous DF versions, it's been a problem that human populations never really reach full potential because of starvation, (early .31 had cities that crashed due to starvation after about 100 years, and never recovered at all) but goblin populations are ones that always fly full-force into whatever population limits are set, since they have no caps, whatsoever. (Further, being populations of Planet of the Hats narcissistic anarchists, most human economic theory is stretched fairly thin trying to be applied to their social structure, which shouldn't be capable of complex society to begin with.) Toady's actually been working to make some cities
not be ghost towns occupied entirely by abandoned shops filled with tables from floor to ceiling.
As I suggested; sites are, under this system, very likely to be reoccupied following their abandonment. I am an archaeology student. It would be foolish to the extreme of me to suggest these sites remain permanently unoccupied.
OK, very likely, why? When it doesn't happen, why?
You have left no mechanism in this model you proposed in the OP to measure these things, utter than blind randomness. Cities rise and fall, are abandoned or reoccupied
for no reason.
This is not true. There are plenty of cases of cities being entirely depopulated, and only resettled after decades or even a century of more. If you read closely, you'll notice I never called for cities to be permanently uninhabited.
You start off by quoting in a goal of making cities abandoned for long enough that soil layers build over the foundations, which outside of major sandstorms, doesn't tend to happen overnight.
Further, why
wouldn't certain sites be completely abandoned forever when their main purpose for existence was depleted? (I.E. why would someone come to rebuild an old abandoned mining town when the mountain has been strip-mined bare?)
Again, the wealth and status of Timbuktu was connected to the gold trade in particular (and salt as a secondary trade), but it has largely fallen into abject poverty after the caravans that were its raison d'etre stopped marching across the desert. (And Timbuktu was on the point caravans chose to cross the Sahara, incidentally, because it was as north as one could get
on the River Niger, which was used to transport the gold from downriver up to Timbuktu.)
That isn't social complexity, it's the simple fact that it was a house built upon a single pillar, and when that pillar is knocked down, nothing is left to support it. Why would anyone want to start a camel caravan across the Sahara in the modern day when shipping is so much cheaper by sea? What does Timbuktu have to offer when you take away its caravans?
It's true that we should want actual socioeconomic rationales as to why DF sites are where they are. But we aren't talking on the scale of specific sites. We're talking about entire societies diminishing or collapsing rather than adapting. Diminishing Marginal Returns is holistically sound, even if it's more simple a system than you'd like. It adds realism rather than detracting from it.
tl;dr blindly accepting Diamond's explanation as gospel when it isn't the academic consensus or in any way considered a defining work isn't something I'm comfortable with. If you disagree with Diminishing Marginal Returns, understand it an refute it.
The problem is, you don't have a system, you have a timer that causes arbitrary things to happen for arbitrary reasons. There aren't "Diminishing Marginal Returns" here, there aren't even Marginal Returns in the first place.
There are no economic processes here with explained mechanics. Monuments are built at one point, and then they can't be afforded the next. Where did the money come from in the first place that it can't be spent, now? What changed to make buildings be subdivided rather than building new housing? Why are plagues suddenly popping up now when they didn't exist when there were more people but less infrastructure in the city?
You don't have a "simplified" version of Diminishing Marginal Returns, you have a
label of "Diminishing Marginal Returns" printed over nothing. I have to go to outside sources to even see what Tainter's theories even
are because they are impossible to discern from your model. If someone can't recognize what's being modeled by the model, then there's something wrong...
I'm not opposed to the general concept of Diminishing Marginal Returns, and in fact, I think it's the sort of thing that really needs to be more prominent in games like Crusader Kings, which do have increasing costs associated with increasingly large realms you have to keep pacified, but it doesn't go far enough in modeling the costs of additional layers of complexity in a single globe-spanning empire. When you hit larger-than-kingdom level, it becomes trivially easy to expand through perpetual war against inferior enemies, and placate any unrest with choice divvy of the spoils of war to favored superdukes and effortless curbstompping of any rebels with a minor detachment of your massive army.
The problem is, again, you're not actually doing anything to represent these increases in costs, nor even where those increases in costs are coming from.
You are saying societies should collapse from
whatever state they are in due to overcomplexity, even when that society may have become stunted in their growth. It's frequently the case that dwarves are stuck in a single, small mountain range, with no place to expand, and as such, they do not ever get the chance to be a giant, sprawling empire that would suffer the problems of a complex society. They are and always were a "simple society" like a Dark Ages European nation the Western Roman Empire collapsed into.
All of Europe wasn't just littered with wholly abandoned cities, not even for just a couple decades during the decline of the Western Roman Empire, but that's what your model says should happen.
For that matter, since wars and starvation and plague are a part of this model of decline, should we just turn off such things until such a time as the time limit on each society dictates they are due to happen? The simplicity of folding everything into a single, self-contained model that does not regard the status of one's environment or neighbors indicates that's actually the proper response.
Further, while it's already being modeled in DF, regardless, plagues and invasions from outside groups are not strictly economic problems, it is merely the capacity of a society to react to those problems that are determined by their social stability and economic vitality. The Mongol Horde is coming in the 14th century whether you have a vast empire with a powerful military, the best technology in the continent, and have the absolute fealty of a legion of hand-educated highly competent vassals or a squalid hut with half a pig shack to your "realm"'s name while you fight with your brother over who gets to own the whole pig shack. One of those is more likely to be able to kick the Mongols back into the steppes, however.
What would make a far more interesting game would be something that actually tries to assess the societal cohesion, raw GDP, and yes, general Marginal Returns of what a society can do, and represent these things in the gameworld, such that players can see the actual wealth and power of a vibrant nation in effect, and see when one site or another falls, and for what reason that fall occurs. (Even if it's nothing more than yet another genocidal elf war.) Seeing the capacity of a society to split itself, (not unlike the Eastern and Western Roman Empire,) so that a failing complex society could turn into somewhat simpler societies that could then survive in some state, rather than just abandoning all the cities in Europe because they passed their freshness date would add far more to the game.
A way to
actually involve these concepts would be to make wood production drops on a per worker basis (functionally costs more per unit productivity) the more lumber resources are exploited. Farming returns are taxed less efficiently, and more rot, the more layers of nobility there are managing them. (This actually happens very well in Crusader Kings - you can only tax one layer down the vassal system, and their income depends upon their own holdings and what the fiefs below them pay up, and non-Islamic feudal holdings by default pay nothing. Burghers are annoyingly free-willed, but at least pay SOME taxes.) Having a transfer of food that is wasted, or in more direct mechanical terms, having an increasing load of food-draining, but not food-producing nobles arise as complexity increases is a reasonable model that would manage to put some burden on a society, and prevent the society's growth from an actually traceable, measurable standpoint. There's a cut for waste, fraud, and abuse. There's a cut for spoilage on the wagon ride to market because villages have to be built further and further from town. There's a cut for the soldiers who have to defend these more vulnerable villages from bandits. You can actually trace the problem to its roots.