I've read a theory that the Tempest Engine could emulate PS2/PS3 backwards compatibility. Didn't Cerny mention the Tempest Engine could do more than just audio? I don't know how plausible it is though.
But just imagine, an all digital PS5 at $399 with PS4 BC, Blu - Ray PS5 at $499 with BC for PS2/PS3/PS4. Would be pretty crazy lol.