At the start there will still be a lot more Mac Pros running AMD hardware that must be supported.
It may not be obvious, but Apple has repair work to do in the pro community. Four years ago this month, Apple unusually disclosed that it was "completely rethinking the Mac Pro." 
This new Mac Pro design wasn't announced until June of 2019 and didn't hit the market until December 10th of 2019. That's just _six months_ prior to the Apple Silicon announcement.
So, unless Apple simultaneously was trying to honor pro users while also laying plans to abandon them, it is hard to imagine that Apple spent 2017-2019 designing a Mac Pro that they would not carry forward with Apple Silicon hardware. Keep in mind, the company had just gotten through a major failure with the Gen 2 cylindrical Mac Pro design.
The current, Gen 3 2019 Mac Pro design has the Mac Pro Expansion Module (MPX). This is intended to be a plug-and-play system for graphics and storage upgrades. 
While the Apple Silicon SoC can run with some GPU tasks, it does seem it does not make sense for the type of work that big discrete cards have generally been deployed for.
There is already a living example of a custom Apple-designed external graphics card. Apple designed and released Afterburner, a custom "accelerator" card targeted at video editing with the gen 3 Mac Pro in 2019.
Afterburner has attributes of the new Apple Silicon design in that it is proprietary to Apple and fanless. 
It seems implausible Apple created the Afterburner product for a single release without plans to continue to upgrade and extend the product concept using Apple Silicon.
So, I think the question isn't if discrete Apple Silicon GPUs will be supported but how many types and in and what configurations.
I think the Mac Mini will remain its shape and size, and that alongside internal discrete GPUs for the Pro, Apple may release something akin to the Blackmagic eGPU products they collaborated on for the RX580 and Vega 56.
While possibly not big sellers, Apple Silicon eGPUs would serve generations of new AS notebooks and minis. This creates a whole additional use case. The biggest problem I see with this being a cohesive ecosystem is the lack of a mid-market Apple display. 
Apple designed and released custom hardware that used a new slot to accelerate compute. My point is that this illustrates Afterburner as a product shows clear direction for Apple to put Apple Silicon into discrete graphics or other acceleration compute in the Mac Pro.
You say that like Apple doesn’t do stuff like that all the time.