Honest question - what areas could macOS running on Apple Silicon SoCs benefit from being able to access the latest NVIDIA and AMD video cards?
Likely answer: assuming the software got ported to ARM64, they'd perform
almost as well on GPU-heavy AI training and serious 3D as a Xeon or Threadripper workstation running the same software on the exact same GPUs. The real question is how could an Apple Silicon Mac Pro tower with PCIe GPUs distinguish itself? The Unique Selling Point of small-form-factor and laptop Macs is power/performance, not raw speed - bolting in a couple of big, sweaty NVIDIA space heaters, dumping the SoC approach throws away the advantages of unified RAM will throw a lot of that under the bus. ARM will likely always be a bit more power-efficient than x86
but who actually cares about that in a
high end personal workstation? The huge-form-factor Mac Pro certainly ain't a battery-powered laptop, nor is it a high-density computing system (where the power consumption and HVAC costs start to become serious).
The main market for the Mac Pro
since 2019 has been users locked into a MacOS-only workflow - which will a shrinking corner of a larger pool which is
also shrinking as laptops/small-form-factor systems get more powerful at one end and on-demand cloud-based processing power for "heavy lifting" becomes more prominent at the other. Not really where Apple would want to throw its money.
Not to mention Apple making the Mac Pro market so uncertain, with the OG Cheesegrater, the trashcan and the 2019 MP each being left in limbo for years followed by a drastic workflow-breaking change in direction.
They could, but it would have worse memory bandwidth, for a start. Their SoC / RAM on package approach affords them unusually good latency and bandwidth.
Well, yes, but the demand for the "Real Mac Pro" seems to be from people who want a
lot more RAM than on-chip unified memory can offer (not just the doubling you'd get with an ultra) and/or want discrete GPUs (which break the unified memory model anyway) - you can't have it all. As I said, its probably not a good idea.