Lets face it, when it comes to a game that's so reliant on perfect clock timing, it's going to crash or misbehave badly pretty much right away.
And you know this due to...?
Dark Souls for example would only show problems with 60fps when descending some ladders.
Look, even if only 2% of games exhibited issues, do you not understand the shitstorm that would create when you're introducing a new console that's not designed to completely replacing the existing one? All you would hear from the press is that "The PS4 Pro
may not work with your games!" It doesn't matter if it was fewer than 10 games in total, as soon as you introduce the slightest hint of compatibility issues it would be a PR disaster.
Again, a title like UC4, with arguably the most talented PS4 development team around and likely one with the most access to the hardware for the longest time, still had to release a patch to get it to take advantage of the Pro, and they couldn't even do checkerboard rendering. It went from 900p to 1080p in multiplayer, and from 1080p to 1440p in single player, with no change in framerate.
That's the creme-de-la-creme of PS4 developers with the most immediate access to Sony engineers.
But really, it's easy to make games run unmodified with benefits. I know because I really want it to be so.