Threadripper is expensive by consumer standards but you can still buy a 32 core 2990WX workstation with 64GB quad channel RAM, 1TB NVMe SSD, etc. for less money than a base model Mac Pro.
Presumably the next gen Threadripper platform with PCIe 4 will launch not too far off the time the new Mac Pro does.
As has already been pointed out, the base model is pretty much irrelevant and that Threadripper system would probably be maxed out. Also that systems wouldn't run Mac software.
Everything you say is entirely true, but most people buying these new Mac pros simply won't care because it's not relevant to their actual requirements.
Presumably the next gen Threadripper platform with PCIe 4 will launch not too far off the time the new Mac Pro does.