Firstly, I want to quote Toady himself.
The long-term goal is to create a fantasy world simulator in which it is possible to take part in a rich history, occupying a variety of roles through the course of several games.
The main reason people enjoy fantasy worlds is because of the stories contained within. Nobody would be particularly interested in Middle Earth if all that happened after Aulė created the Dwarves was that they settled down to work on Moria, happily ever after, until the end of time. That's not a story. It can form the
background to a story - Dwarves working on Moria in the face of orc attacks, proletarian Dwarves struggling on building Moria under the oppression of the Dwarven nobility, etc - but fundamentally the core of almost any compelling story is a conflict, real or metaphorical, to be solved by the protagonist(s). If Dwarf Fortress generates worlds absent of meaningful conflict, it has missed the point of what a fantasy world simulator
is. Yes, Dwarf Fortress is a sandbox insofar as one defines one's own goals, but those goals should be achieved in the face of struggle, not tedium. Currently, a greater degree of difficulty is therefore necessary to make Dwarf Fortress a good fantasy simulator.
But fine, you may disagree with this. So secondly, I say that you make far too much noise for such a small issue. Let's run through some commonly proposed suggestions for making Dwarf Fortress sieges more enjoyable
1. Allow lv.2 building-destroyers to destroy bridges.
2. Allow lv.2 building-destroyers to destroy fortifications and constructed walls/floors.
3. Reduce the relative gains from underground farming by some amount, either by requiring more tasks as part of planting [fertilizing/watering], reducing the amount of crops produced, reducing the speed at which crops grow, reducing the seasons in which any given crop can be planted, or making dwarves require more food. [I make no specific suggestion and any of these has strengths and weaknesses]
4. Make non-grazers also require food.
5. Reduce egg clutch size.
6. Make foods perishable.
7. Set goblin military skills to scale with fortress wealth.
8. Allow certain enemies to dig soft soil.
None of these fundamentally change the sandbox-satisfying capacity of Dwarf Fortress. If you are interested in mega-projects, then by embarking on an island away from goblin sieges the military changes are entirely irrelevant to you, and the only changes you have to worry about are the changes to food and changes to building-destroyers. The changes to food distract from your ability to pursue mega-projects only in a very small way, as by the time you are usually able to focus on mega-projects you have at least 80 dwarves, and if you have at least 80 dwarves, 40 of them will usually have nothing to but haul anyway. Taking another 20 of those to bolster your agricultural industries will not be easily missed. That means, barring the change to building destroyers, these changes have almost no impact on your capacity to pursue mega-projects. However, they have a big impact on enjoyment for those of us who thrive on conflict. It is completely nonsensical to oppose them as such; you are wilfully denying Dwarf Fortress a larger community for no real reason.
These are small changes, they don't affect you particularly but others would enjoy them very much. There's a ~50 page thread on improving farming, for example. Why do you put up so much resistance?