We reached diminishing return regarding game possibilities at the end of Gen 7, then Gen 8 was just an incremental improvement and then this gen not even that... If you mean in graphics, well, once PBR was mastered, we reached diminishing return until ray tracing costs around the same than rasterized lighting when we can see a big jump (but after that I don't see any more important visual leap).
So Gen 9 games can be the same than Gen 8 games, I wouldn't expect any better than that mostly due games costs exponential increases.