a bit in reverse order....
1. That this has only extremely tenuous linkage to the topic of Mac Pro.
2. It is grossly bad idea that is far outside Apple's past behavior or acquisition strategy.
[ And no, the PowerPC, and ARM design shops they acquired are not similar in a specific sense; only hand-wavingly similar. ]
3. Likely will generation more Nvidia fanboy vs AMD fanboy tit-for-tat thread contributions than anything insightful.
Apple could acquire AMD for a few billion dollars and own their own desktop x86 CPUs and GPU technology rather than paying a premium for Intel CPUs (and either AMD or Nvidia GPUs).
The question largely unanswered and deeply unmotivated is whether Apple needs its own desktop CPU/GPU. Why can't they use what is generally available and funded by the overall market? Control just for control sake is silly. Apple doesn't really practice that.
I guess part of the reason is that AMD just doesn't have a competitive x86 product.
A minuscule part of the reason. Even if it was more competitive (it is likely competitive enough to get some concessions out of Intel ), it doesn't answer any strategic objective question for Apple.
Just because Apple has billions doesn't mean they have to blow it. They have the money because the investors expect them to invest it in a way to earn a better than return. Buying AMD doesn't what? Apple already has about the highest industry average for its portfolio of classic laptop/desktop product line up. If Mac profit margins were collapsing maybe there would be a strategic/tactic reason, but they are not.
I'm also guessing someone at Apple has done the math and it's cheaper in the long-run to just buy these components in volume rather than own the R&D and manufacturing.
The math is more so about sharing R&D and manufacturing costs with others.
Or perhaps, they see their own ARM chips pushing up into laptop territory (where the volume of Intel chips are used) and thus x86 is not the future.
It is doubtful they are drink that much Cupertino Kool-aid. Apple's ARM chips are substantially customize to run extremely well in iPhones and iPads.... not laptops. Better than laptop territory from 5+ years ago, but not necessarily better than what is available on future product roadmaps.
ARM designs are magically better thermally just because they are ARM.
http://arstechnica.com/gadgets/2015/04/in-depth-with-the-snapdragon-810s-heat-problems/
Sure the 810 would do incrementally better if a tablet or chromebook like enclosure, but it also indicative if they try to push the envelope closer to where Intel's CPU largely are now they have the same kinds of issues to juggle. The general architecture that ARM uses is not a magic bullet.
Apple has a 3 , not 4-8 core design. For a phone/tablet that runs one application (along with strictly limited background tasks ) at a time that count of 3 is plenty.