A Threadripper is equivalent to a many-core version of Intel's HEDT processors or 2xxx series Xeon-Ws, NOT the W-3xxx or Xeon Scalable series. Notably, it doesn't have the enormous RAM capacity of the W-3xxx chips, and many people who are after a machine as powerful as the Mac Pro actually want more than 256 GB of RAM.
What AMD has done (successfully) is nearly double Intel's core count (while maintaining comparable TDP and turbo speeds) at a given processor level. The Ryzen 3950X is a scary-fast standard desktop CPU (it still has dual-channel RAM and no ECC), not a true HEDT CPU that supports quad channels and ECC. It's a much faster competitor to the Core-i9 9900K and the new 10900, not to the Xeon-W 2xxx CPUs in the iMac Pro. The Threadripper 3970X is a scary-fast HEDT/entry workstation CPU with a 256 GB RAM capacity, not a high-end workstation CPU that can handle a terabyte of registered RAM.
At both the desktop and HEDT/ entry workstation levels, AMD is well ahead of Intel - it's not especially close except in weird workloads that really use the latest AVX extensions. Both the top Ryzens and top Threadrippers are so fast that it is tempting to compare them to Intel CPUs a level up (and they hold their own in raw speed) - but they don't have the features of CPUs a level up.
AMD does make EPYC CPUs that compete with the W-3xxx and Xeon Scalable CPUs, supporting tons of RAM, multi-processing and similar features. They are a tradeoff with their Intel competitors (AMD will certainly give you more cores, but, unlike Ryzen and Threadripper, they'll be slower cores, with lower base and turbo clocks). If your workload is well-threaded, EPYC will crush Xeon Scalable, but the Xeons have an advantage in poorly threaded workloads.
Intel lowered their pricing to compete - the i9-9900K is slower than the upper-end Ryzens, but it's also cheaper. The W-22xx line comes in between the pricing of upper-end Ryzens and Threadrippers. AMD is still the better value, but not by as much as it was at Intel's old pricing.
If Apple were going to go AMD on the desktop:
Mac mini, 21.5" iMac - Ryzen 3 and 5 with integrated graphics - CPUs comparable to or a little faster than what they're using now, GPUs a huge improvement.
27" iMac - Ryzen 5, 7, 9 with discrete graphics - lower end CPUs at least comparable to what they are using now, upper end a huge improvement (close to double!).
iMac Pro - Threadripper - not enough models (only 24 and 32 core options) - both a huge improvement over Intel equivalent.. Apple would need a custom 16-core 3rd Generation Threadripper (can't use a Ryzen 9, because it needs a completely different motherboard) for the low-end model. The 24-core Threadripper 3960X costs twice as much as the 8-core Xeon Apple's using, and the new version of the Xeon is cheaper...
Mac Pro - EPYC - many more cores, but slow single-core performance...