I have used mac portable, quadra 700, G4 just to name few and compared with was available at the time they were something else. When PowerPC did struggle against Intel regarding Power, Performances, etc... Apple did not have too many options but to go to Intel. This is where Innovation took a beating since you had to be as everyone else. Granted you can tweak but still....
They can always use AMD to boost performances in the short term but to be 'Apple=innovation' again ARM would be the best route. After all Apple uses this platform for years. I do not have access to their R&D but I'll bet they have OS X running on ARM. The other would be Risk-V but this is another story.