Imagine PS5 being backwards compatibility to all Playstation consoles, damn that would be sick!
Well it would be possible, PS5 should be powerful enough to software emulate the PS3. But it won't happen.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)