That would only be useful if AMD were to have a great design team that could be kept together.
Also, EU competition law would require your hypothetical Apple/AMD to sell CPUs to PeeCee makers at competitive practices, which is a hassle that Apple probably don't want.
Anyway, I hope this will be the end of the tiring comments in every thread suggesting that Apple should have used AMD. Apple tried AMD and found AMD unworthy. End of story.