If you'll forgive the pun, that's not an Apples to Apples comparison. The tradeoffs and commercial incentives are quite different.
Apple (and Android) has a community of third party developers building technically simple apps that can tolerate the carefully managed shifts in the iOS development APIs. When a breaking API change does occur, app developers (who are mostly on an equally carefully managed continuous delivery cycle) rush to work around it and release updates.
By contrast, Cell was a noble innovation that at the time Sony and IBM thought might be practically useful for general purpose computing: office workstations running a variant of Linux etc. That didn't really take off and now it has to fade into the background. Sony is obliged to "fail well" here and keep hold of developers by not proposing another high-investment, short-lifespan development platform.
Unfortunately at the C++ development level Cell is like chalk and cheese compared to a more standard multicore CPU / massive GPU architecture.
I'm sure a backwards compatibility layer could be built, which either allows PS3 games to run natively, or with minor porting, but there's no point announcing it if it's not certain or complete, and it would only be economical if a significant proportion of new discretionary spending from users was expected to flow as a result.
In addition, the majority of major game releases contain their own internal compatibility layers which allow them to run on different hardware and be sold on different platforms - but the platform vendors don't control that, or how it works.
The most important decision for Sony is to stop chasing Cell or other exotic architectures, and that's a good call. The rest of the offering has to be conditional for solid commercial reasons. I'd suggest it's more up to game developers to port their older releases to the PS4 and sell them as DLC, perhaps at a nominal price to existing owners. The possibility of more and more incompatible consoles proliferating in everyone's loungerooms is certainly a bit odious.