In this week's bombcast the GB crews talk extensively about backward compatibility of PS4, while Sony is trying to achieve compatibility in long term with cloud and streaming, I had different a thought when watching PS4 meeting:
Why not produce a hardware/software addon that enables you to "delegate" calculation to the PS3 Cell processor - then you can attempt to wrap the PS4 interface and controller around old PS3, with only drawback of having to power PS3 at the same time.
If you really care so deeply about compatibility, then I guess paying extra for that compatibility isn't a big deal? It would work better than the cloud (as local and native hardware, much like the recent Nvidia solution). And it lasts for as long as your PS3 is meant to last.
Eventually some sorts of emulation may come out to replace PS3 hardware (if people care enough), or out-sourcing everything to the cloud when it's really ready.