It's not just the hardware speed improvements that are slowing down but also innovation in the games industry itself. Really early games can be a bit hard to get into because conventions and features that are taken for granted today weren't established yet. But that is already much less of a problem for a decade or even two decade old game. Games made in a decade will probably be even less different from games made today. That means that future gamers will have much more existing games to choose from that they can easily get into. Games which all compete with new releases.
Essentially the games industry is no longer the fast moving new thing and now has to deal with it becoming harder and harder to top its own classics.
Essentially the games industry is no longer the fast moving new thing and now has to deal with it becoming harder and harder to top its own classics.