Unlikely.
' ...
I ask him what Apple’s philosophy on external GPUs is.
“I think they have a place,” he says, and leaves it at that. ...'
https://techcrunch.com/2017/04/04/apple-pushes-the-reset-button-on-the-mac-pro/
I think Apple may grudgingly enable this, but it seems that this is not a hot issue for them. Once the GPUs on are on the other side of the TB bus and sitting in standard PCI-e physical slots, that is going to hard to keep a AMD ( ATI hasn't existed in years ) GPU only solution. Or even a "build for Macs" solution.
Apple's macOS GPU stack is in enough turmoil as it is with leaving OpenGL/OpenCL/ open standards for Metal anyway. I doubt they want to stack detachable GPUs to the workload.
Modularity here appears to be the ability for Apple to put a new GPU system of their own into a container that they themselves designed. It would be also easier for them to build custom cards at a low resource consuming rate and on a regular basis if just had more flexibility of putting card and cooling into a module slot. No coupling to CPU, industrial design group , Thunderbolt updates , etc. etc. Just a GPU subsystem of their own limited resourced design.