In response to the original post:
It's a phenomenon I can understand - If the DM doesn't want to, or they don't have time to do a LOT of work creating a believable campaign after the PC's have succeeded in world-changing changes, then they don't give the PC's the option to change it. However, if that's the goal the players set, they become very disappointed to find out they never had a chance in the first place. Very few people like tilting at windmills.
If the DM has the drive to let it happen, however, and the resources to create a consistent world in which the players will play in AFTER the changes, then they do their players a disservice just because it would "change the flavor." It is, after all, there for everyone's enjoyment, not just the DM's. The published campaign wouldn't change one bit, and the only people who care about this alternate history would be the players and the DM. The DM has every right to set the bar high, but to be fair, it should be attainable.
But from a standpoint of not feeling capable of it, or not having the resources to do it, I can understand.