Of course emulation requires a very complex software... But the PS4 APU does have more than enough power to easily emulate the two general purpose threads of the PPE and the parrallel threads of the SPEs. The theoretical raw performance of the entire PS4 APU is well over 2 TFLOPS including the GPGPU capabilities of the GPU, compared to the 0,2 TFLOPS of the PS3 Cell.
Sony wanted a streaming and steam-like download service including social features, so they bought Gaikai. I seriously doubt that Sony bought Gaikai just for emulation, although it is quite possible that this affected the decision IMO.