My money's on the "spark of awesome" outcome. There's only so many ideas a set team can pack into a game, and it certainly looks like they've got almost all of them in there already. Considering, as Max demonstrates, a dozen or two hours of gametime and some luck is all a person really needs to have every item and see every boss, and you can even keep your stuff from one gameworld to the next. If they don't crank out awesome shit on a weekly basis, "no modding" is going to mean "no replayability" for 90% of their customer base.
Of course, since the cover charge is all they ask, why should they give a shit if you're bored after a few days?
Anyway. Broke my first Shadow Orb, and got a musket with no ammo. Oh goody. At least I know where some Demon Alters are now. And Eyethulhu's back. I swear to God, he shows up every other night cycle, and there's not a damn thing I can do about him.
Well, like I said, the source code
was not built to be modded, nor was it made in a way that you could make it moddable. Not in the MC way where you give out source and let people go for it, nor the DF way where the game loads up data from raws. All items are hard coded, all in a single class, with no inheritance so you don't have a super class to extend from when you want to add your own item. It's a nasty piece of code. So to fix this and allow for modding would require a major overhaul, something I don't think Redigit has in him, from what I have seen. Modding just will not happen here, it is a pipe dream.
As for adding new content, well they might be able to keep it up for a while. They have a large fanbase all throwing out suggestions, and from that we can expect something interesting. They also haven't limited themselves to a genera, as we are seeing metal swords and plasma guns, so it's an 'anything goes!' rule, adding to the number of things they can add. Expect steampunk gear.
But I wonder how manageable that is. The reason that design patterns in OOP systems are so very,
very important is so that your code don't turn to spaghetti as you add new functionality, and the terraria code is already spagetti, so adding new content is likely to become more and more glitchy and take more and more time.
So yea, expect a spark. A wonderful, magical spark, that it would be a sin to miss, that will bring joy and mystery, but a spark. Redigit programmed himself into a corner, so I would say that a year from now the jig will be up.
Then again, Toady doesn't have programming qualifications, he could be working on spaghetti right now, and doing alright at it after seven years. But then again, he is a very smart, well educated man, and he has been programming larger scale games before DF, so he is more well versed in what to expect. He probably planned a bit further into the future.