So why not just have it for games that do work. A whitelist or blacklist system?
I guess the process for testing the games is the real hold up. Simply patching a game to be in "PS4 Pro" mode to do something like bump resolutions from 900p to 1080p (ala UC4 multiplayer) or unlocking framerate (ala Rise of the Tomb Raider) should be trivial in most cases... I'm guessing nobody really wants to spend time on old games with little benefit to them (the publishers and Sony, not us). If all 360 games worked on XB1 for free, in the exact state they were on 360, I can imagine that's what we'd be left with. We're seeing the benefits of work being required to make them playable at all.