I mean PS3 had full PS1 support and they already have a working emulator on PS4 for PS2. PS3 I think is possible but I doubt they'd want to put the effort into a good emulator. That said I only expect PS4 BC.As it stands you should only expect PS4 BC. Everything else is just a cherry on top. Although there's no real reason why it shouldn't have PSone/PS2 support as well. The PS3 along with the PSP/Vita are the real question marks.
PSP/Vita would be limited by its smaller digital library but could you imagine if it had a UMD drive and Vita card reader? It's a dream but what a dream! Crisis Core playing on your PS5.... Mmmmm.