Maybe they could just say that compatibility with the S is optional next time. Yeah, eventually there's going to be a gap of some sort. Still, what about less demanding games? I don't think those developers should be locked out, either. I say, let the developers decide historical support.
It'll always be optional though and always has been after a gen changes as far as I know, they aren't going to tell a dev not to do it. The problem is for people who buy a One X or PS4 Pro thinking they are future proof, these machines still have outdated CPU's, they are just allowing us to play prettier versions of the same games the base consoles were already capable of playing.