I don't have a reason to buy a mac pro that isn't intel based, but after they burned the Intel bridge (& the Nvidia bridge before that) there's no chance. To satisfy both companies it'd take forging some kind of weird hybrid Intel + Apple Silicon homonculus in the fires of deepest hades, and there's just no way it's worth the effort to Apple. Look at their revenue streams. iPhone, services, etc... then there's the lowly mac trudging along, and the pro a tiny sliver of that. Meanwhile Apple's been beating a "post-pc era" drum since 2010, pretty upfront about the mac in general being overkill for most people/most of the time. The company Apple has turned into would be wholly justified just spending as little money as possible selling the least amount of hardware possible to the uncritical masses for as much as possible and raking in shareholder gambling $$$. The mac pro, and probably the mac entirely, fit into that scheme less and less with each year, and each new product category that they'd rather sell.
For me, Apple is now a good phone/watch/AR/VR headset company at this point. But for work, I guess it's going to be a Surface + Nvidia/Intel workstation.