Even if they use DLL modules for PSSR, the way developers have to patch themselves their games to the latest version of PSSR should mean the DLL is "shipped" with the game, right? So PS6 wouldn't be able to replace PSSR with FSR4 by a system level swap?
I think the ideal solution for PS6, which I assume is in what Sony an AMD are working on, it's to not only just to improve PSSR, but instead to move it to a OS/system level, integrated in how the console renders the visuals, so they can apply it to all games rendered there (including PS5 and PS4 games, including the PS1/PS2/PSP Classics), being able to boost resolution and framerate of any game ran there without needing any per game PSSR implementation in the game's code.
If they achieve this, they could have a portable underpowered hardware compare to base PS5, who would use this to run base PS5 games natively running at the same or lower resolution than the commonly played in PS4 or PS5 (let's say internally works like a PS5 in low power mode limited to 720p-for when in portable mode- or 1080p -for when in docked mode-) and then use this to bump to 1440p -for when in portable mode- or 4K -for when in docked mode- by doubling its width an height, plus pretty likely also bumping fps with an optional 60fps cap.
I assume this would allow them to run any -or almost all- PS4 and PS5 games at 1440p in a portable at 60fps, maybe even the one locked at 30fps, at least in my fantasy. Now it's just a fantasy, but I trust people like Cerny who make stuff that are almost a miracle.
Regarding PS6 games, which I asume all of them will be released for PS5 to run them in the portable and also to take advantage to people who will stay on PS5 years after the PS6 release, would be able to enjoy this without having to spend game resources or dev time on it. Meaning, they may natively do their games with a native 1080p resolution and lower framerate, to focus the horsepower on adding extra detail and more dense worlds, and then use this next-gen, OS level PSSR bump losslessly the resolution from that to 4K or even 8K and also bump their framerate.