If designing an ARM CPU for the Mac Pro alone made no economical sense to Apple, I guess they could perhaps source a high-performance ARM server CPU from someone else. Marvell’s Thunder X2 for instance has up to 32 cores/128 threads and supports up to 2TB of RAM.
Apple wouldn’t do this. Their custom chips have so many differences from the competition that their flagship Mac would lack many key features their chips are known for that all their other macs would have. Their flagship Mac needs to have the best features. The Mac Pro is the prime target for some insane machine learning cores and that will be a huge selling point.