Going to go against the grain and say that part of the reason why the Nemesis system never took off wasn't just the patent; the patent didn't get approved until last year, and was rejected once, and any developer that really wanted to could have probably found some way of doing it that went around the method described in the patent but accomplished a similar effect. I think the real reason is that the Nemesis system is harder to balance and much more limited in what you can do with it than most people admit.
One of the biggest knocks against the original Shadow of Mordor is that the game is so easy on its default difficulty that many players never really got those stories about rival orcs who had killed you and got promoted up the ranks to be your nemesis in the final fight, so the big thing you got was pop-up notices that randomly generated orcs were replacing the ones you had killed, which is neat but not really worth designing a whole game around. Speaking of which, that's what I meant by limited: unless you're specifically making an open world game (which take a long time from preproduction to release) in which your character is constantly defeating opposing members of a large rigidly hierarchical organization, then the Nemesis system isn't something you can just bolt onto a game.
