Luke7
because they are highly customised APUs… Just because two generations are made by the same provider does not mean they would be remotely compatible. That is like saying XboxOne games should work on PS4 because they are both AMD APUs…
It's not like that at all. It's quite likely that PS5 will be based on similar hardware to the PS4 (looks like the days of fancy custom chips are over). If that's the case, it's extremely likely that the APIs for the PS5 will be based off the the APIs for the PS4. Backwards compatibility will be a breeze, between those machines. PS3 and PS2 emulation is a different kettle of fish. They have complex custom chips that need calls to be intercepted and translated. PCSX2 is the most complete PS2 emulator, I know, and even that doesn't always get it right, on powerful hardware (Virtua Fighter 4 has noticeable slowdown on my Ryzen 1700 with Radeon 580). PS3 has, a NVidia based graphics chip, although that may have very different APIs to the PS4, but the Cell processor is also a very complicated beast to emulate.
You could, in theory, put a Cell CPU inside a PS5 and use it as a co-processor for other, more menial tasks, but then use it as the main CPU to run PS3 games. That'd remove the need for the PS5's main CPU to emulate the Cell. After all, the PS2 did something similar and used the PS1 CPU as an IO processor; utilising it to run the PS1 games. But a lot of that depends on if they'd continue to be produced, and the cost of a CPU.
But PS5 emulating PS4 IF based off same hardware, will be a doddle.