The money spent on putting a cell in a PS5 would be better spent on a HBM die, or some small SSD to cache textures, or something.
You did see that even the example you presented shows this, right?
You expect a PS5 to have ~6 times the GPU compute of a PS4, and the PS4 can do ~16 times better in the example...
All on the GPU.
The task you gave is well suited to a GPU, and the cell does well in that kind of workload, but the PS4's GPU mops the floor with the Cell.
Even if it only costed a dollar to put a cell in the PS5, per unit.
That money would be better spent on almost anything else.
The money spent on R&D to make such a configuration possible and feasible (both hardware and software wise), would be better spent on Anything else.
The PS5 should be able to handle PS3 emulation just fine without a Cell.