The only reason to hold on to Cell is backwards compatibility. It was and still is a horrible decision as far as CPUs go for a game system. It may be good for scientific processing or video decoding, but it just seems ill suited for something a branchy as video games.
Here's a question for those more knowledgeable than myself. Would a PPE with a local cache be capable of emulating an SPU, or more concisely, does the PPE support the same SIMD instructions that the SPU is built for? Would a custom solution that included 8 PPE(2?)s be able to emulate a Cell? If the PPE supports those same SIMD instructions, then it seems like a somewhat elegant solution to the backwards compatibility problem. Past that, as long as the GPU is capable of the gymnastics it would take to emulate the PS2's GPU, then the entire history of the Playstation brand is there. Pop in a disc, or download it from PSN.
Alternately, PS3 backwards compatibility could be completely forgone, but for the fact that it's an amazing hook for prospective buyers. You can still play your old games; they'll load faster and may even look better. How many truly iconic games does the PS3 have that aren't 1st party? Could updated executables be provided for certain selected games? If you could run MGS4, FFXIII, Uncharted 1/2, the GoW games, LBP, and Trico, would that be enough?