That doesn't make sense though. I can't see them releasing upgraded hardware then going "oh but it doesn't work on these games, it only works on these ones" "these games might not work on the old ps4/xbox one, only new models" then saying "Oh and on the games it does work on, it'll only work on these certain modes, not the full game".
Then those that do buy the upgraded consoles complain because they're being intentionally held back for parity, I mean it already happens with PS4/Xbox one/Last gen. It comes to a point where there'd be no point releasing upgrading hardware and they might aswell jump into next gen instead, and restricting it like you're claiming is way past that line in my opinion. I really can't see them making SP 4k then once you hop into MP it's toned down to 1080p, 900p or even 720p. There's a gigantic difference between 1080p down to 900p and 4k down to 900p.
In addition when has something like that ever happened? Phones, Tablets, and PC gaming the improvements are across the board not in specific areas of games/apps on upgraded hardware, why would consoles be any different in this regard? It'd be much easier to just copy how it works on PC since that's more or less the route they'll be going at that point.