Really, there are not a whole lot of different game genres when you get down to it. If one had a good RTS engine, for example, you could build a WWII simulator or a sci-fi setting on top of it with only a few mods plus new unit models. And from a player's standpoint, there is something amazing about playing a game that has been developed for decades like DF or ADOM, because it is so much deeper, more detailed, and well-balanced than the average new game. And has less bugs. I think this thinking is behind what must be a massive effort to reskin Morrowwind. The increase in modding's popularity is another indicator of this trend.
Of course I doubt new games will entirely cease to be made. There are certainly genres that haven't been discovered yet. But it seems that the bulk of player-time is moving gradually towards games like this IMO.