They could also just pick up where they left with the Xserve:
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We had an XServe at work, and to this day I think that was the best-looking piece of rackmount hardware I've ever used. Also a good design internally, but there is other stuff that's on par on that front.
It's good (for pros) that Apple is going to keep Intel chips in the ultra-high-end hardware for as long as it makes sense--a forked architecture is a bit unusual, but I appreciate them sticking with what works.
That said, two M1 Max chips in one computer would probably be competitive with a current-gen, top-of-line, 28-core Xeon CPU or an AMD Threadripper (impressively, at less than half the power of either, though that's not very important in a desktop). Four of them would be something else entirely.
It's the GPU and RAM that are the bigger questions to me.
If Apple continues to not support 3rd party GPUs in the eventual AS Mac Pro, even four M1 Max equivalents is not going to rank in the same league as dual WX6800X Duos with 64GB RAM each (heck, dual ones wouldn't even have as much RAM as the cards once the system memory was deducted). Maybe Apple is willing to cede the extreme top of the line, but if they sell that kind of absolutely ridiculous hardware,
somebody must be buying it.
An on the RAM side, 1.5TB is a lot more than the unified memory is going to do, and even if they're willing to put that much on the die, it's a lot to ask a business to plan in advance for $22,000 in RAM at the time of purchase, rather than upgrading as needed.
Plus the ECC question. I'm assuming that the current M1 architecture does not do something equivalent to ECC internally.
All of which is to say the desktop/pro architecture is going to be a lot different than the ultra-integrated mobile/lower-end one, or they're probably going to give up on the HPC end of the market.