We know why they won't cause Apple is so focussed on efficiency unless they get rid of that goal it's never going to happen.
Apple's GPU cores are slow as well they are clocked lower.
Clocked faster isn't necessarily a bad thing. There were multiple generation where AMD tried to make up the deficit to Nvidia by just ramping up clocks. It got AMD hotter GPUs , but didn't necessarily win the top end performance mark. ( If clock past what the underlying fab process it tuned to ... then tend to get quickly into the zone of diminishing returns. )
Apple's GPU go 'wide' , high compression , and cache heavy instead of higher clocked. When AMD and Nvidia go to N5/N4 that cache gap is likely to close. That is part of the issue. Most of the 'performance levers' that Apple is pulling aren't exclusive to Apple GPU. ( compression schemes between desktop and cellphone different , but general technique is used in both places. AMD and Nvidia have had caches ... just not as big ones. ).
Apple has pushed wider memory channels down into the lower end GPUs more than AMD/Nvidia. That probably won't change. Apple is probably on the relatively more expensive path and the other two have cost and market segmentation concerns to handle. But at the top end of the GPU spectrum, Apple's GPU memory bus is not particularly wider than the top end GPUs (that don't have to devote transistor budget to CPU cores and share memory bandwidth. ).
Apple's GPU doesn't scale up as well and doesn't scale out at all. As long as in the lower 60-80 of performance range they do OK. It is the upper edges that are problematical. However, for. 80+ % of what Apple sells ( laptops and low-midrange desktops ) that will work.
Now with Windows out of the way cause of no support from Apple or MS. The Mac Pro will once again broken if Apple only allows macOS.
It really isn't "Windows". It is closer to the situation where the initial Intel Macs had no BIOS boot support ( and were EFI only). Only worse this time because not even standard EFI is the boot layer now. It is all iPhone iBoot and a highly limited MacOS 'one true boot" adaptor layer on top.
It is about a 180 degree turn from where the Mac Pro 2019 and 2017+ Macs had arrived at with UEFI AMD GPU card support. On top of that, prominent feature now is running native iPhone/iPad apps that are hard wired to expect an Apple iGPU.
There is a big disconnect down at the low level foundational layer. Windows possible reglulated to virtual machines is only symptom of the issue; not the root cause itself.
When Apple switch to Intel they jumped into the Intel ecosystem ( and Intel was trying to push EFI along.... in part hoping that Apple would help drag some of the other system vendors into the "new world". UEFI did as much to keep BIOS going and help move folks along. ). This transition the "huge native ecosystem inertia" is iPhone boot.
With no AMD support the Mac Pro is dead to me unless Apple makes dGPUs that are on par or faster than AMD.
In the low to a healthy portion of the midrange they have already done that with an iGPU that you will have to buy anyway.
As for an Apple dGPU. Again foundational changes. All the extremely highly tuned Apple GPU apps expect an iGPU. So is Apple going to develop and another driver stack to do dGPU and tell folks to go back and redo all of your optimization work again. (after just asking folks to do major overhauls for iGPUs)? Probably not. At least not for a couple of years.
More likely Apple will be after app developers to optimize for the Ultra (and what will effectively be a dual ultra ) GPU that has some non-uniform memory effects have to work around rather than chasing a dGPU. Also whatever upgrades they add to the Apple GPUs in generation 3 , 4 , 5 of M-series ( and A-series). [ more VR/AR stuff , new compression , etc. ]
So I want Apple to make another Intel Mac but we just have to see.
At this point... probably more likely to get new MPX modules for the 'old' Mac Pro than get a new ( W3300/W3400 ) Intel processor. There are 6x50 version of the GPU that don't really need any major driver work at all. And if stick with single die RDNA3 probably not a large effort to update the drivers.
There is a recent thread where someone is bragging about a Geekbench Metal score with a 6900 stuff inside a semi-hackintosh Mac Pro 2009 beating a M1 Ultra. Same thing, but less hackintosh, could be done with the MP 2019 chassis if iterate another generation on AMD cards. Apple is
still selling the 2018 Mini along side the M1 Mini. They could just keep going on the MP 2019 for another one or two years if just selling to the most expensive possible GPU focused crowd. And do a "half sized" Mac Pro with non AUX powered PCI-e cards with a Ultra and "double Ultra" for folks that aren't as focused on max budget on GPU cards.
