My guess is that all future games will be made to support this. It's likely that Sony didn't intend to do this initially. The whole "driver level upgrade" thing is still quite a new idea in the PC space. Once Sony saw AMD do it, I'm sure that was when they figured out how to do the same. It's a great feature.
The problem for Sony is that they will need to keep computational costs similar to any future versions of PSSR. Otherwise the upgrade could cause framerate drops in some games. Perhaps this is why it is an optional feature?
TBH, I love the boost modes that Sony's Pro consoles have offered in games that don't officially support the console. This is a nice continuation of that trend. The newer upscalers can make a big difference, so it is great to see PS5 Pro making a big difference to gaming visuals. It really shows how much this generation of consoles has been let down by the lack of a good upscaler.