The genre has gotten really stale, but I still play them if the gameplay/story is good enough to keep me enthralled. The problem is how do you pick a new setting without upsetting the balance, look at for example, The change in pace ACIII has taken, from big sprawling cities with massive structures, to the tree hopping madness of the American Revolution. People are already complaining about the game and they haven't even played it.
The same will happen to any developer who tries to shift the focus of their current sequel to another locale. Severe backlash from a community content on berating the minor points rather than saying well done on X and Y. The post apocalypse setting is great for story telling as you have a basic set up, millions dead because of Disease, Terrorists, aliens etc, you're part of a small resistance refusing to lose your humanity. You know how to stop the impending doom. You become hero. Now insert characters around that and you have a game people will buy!
It's the same as movies, when was the last time you saw a movie that wasn't a remake/rehash of an older film. We've reached a point where creativity is frowned upon and the making a quick buck reigns.
On the flip side, I still buy these games and watch these movies, but I will do so through gritted, hate filled, teeth!
