At this point, the hardware on the PS5 sounds powerful enough to provide backwards compatibility for the PS3 (and all of its predecessors). That would appeal to some people who have old consoles/games collecting dust in their basements/attics.
It would be nice, but I'd be surprised if they make much of an effort. There's little financial motivation to make available a huge library of practically free games, especially if people are willing to pay for new "nostalgia" consoles with a small selection of greatest hits preloaded.
I mean, Nintendo does it, and then makes you buy them again, and again with zero transfer of accounts to new machines.
Don't tease me. Backwards compatibility with with PS3 AND PS4 would be beyond amazing.
Why do that when they can sell you a remastered edition for $60.