Regarding PS3, most of its games are already available on PS4, except a few gems. They could just work on those and make them available on PS4/PS5 or PS Now.
PS2 and PS1 could come back as re-released classics. They could select the most acclaimed games. Having full BC with PS2 and PS1 would benefit some players, sure, but most current gen players wouldn't give a damm. I think it'd be a waste of work. Unless we're considering plain software emulation which we know that will never happen due to many technical reasons that many don't fully understand.
That would be great considering people like myself who bought a ton of PS1 and 2 classics off the PS3 PSN digital store which are now locked onto that specific platform. It only does everything is no longer a care for Sony apparently.