This. In a previous fort, my first 2 or 3 big sieges had goblins, beak dogs and trolls. The 2 or 3 after that only had goblins and beak dogs, and after tht the sieges were smaller and only had goblins. Now, I'd observed that trolls and beakdogs took higher casualties than the goblins did, and were often wiped out entirely. I can only conclude that either I had destroyed their domestic populations completely or had reduced them so drastically that the goblins weren't going to risk losing the entire population by using them as warbeasts. Point is, now the enemy has resources of his own to manage as well, and sometimes that means he won't give you his all, or as much as he used to.
A good DM (Dungeon Master) should always have new challenges ready for his players. This new world simulation should never lead to :
"Congratulations on being so successful! The enemy is now too weak to present any threat forever. Grab the dice, roll a new
character fortress, and start over."
I hope
something is planned beyond that world simulation. It should never lead to getting the upper hand (perhaps totally independently from the player's own actions) and the fortress now being safe forever.
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Why not? As mentioned, DF is first and foremost a world simulator. After establishing a successful fort and managing to "pacify" a region, you go to a new frontier and "settle" it. Infinitely generating enemies out of thin air goes against the base nature of DF, why else are FBs tracked and Titans/megabeasts get lairs? Even werebeasts are individually tracked and come from nearby caves. Personally, I feel like its a sense of accomplishment. The goblins have broken themselves upon your bulwark, and are a pale shadow of what they once were. Congratulations! It gives a sense of accomplishment. *shrugs*
If you really want "limitless" sieges, just go settle next to a dark fortress. You can check with legends viewer to make sure the population is redonkulous, but they are almost always stocked with 10,000+ enemies in worlds with longer histories. Their demons are often still alive too, and can take the field leading armies. Human towns often have nearly the same populations, so if you find their first one you can settle nearby and provoke them (once the human caravan is bugged, or use an older version).