Yes, and 98% of Apple's products will continue to run on their own GPU and Metal API. But what about the Mac Pro? It is like what the Ferrari is for FIAT. A super sportster with technology way beyond any normal car. The Mac Pro is not indicative of the direction the entire Mac platform is moving in. It just needs to be fast.
The uncomfortable consideration is that driver development for the AMD GPUs inside of the Mac Pro was subsidized by the much higher volume graphics chips inside of the iMac and MacBook Pros. Most of the folks who want GPUs to fit in slots for an Apple Silicon Mac Pro want to use affordable third-party cards, which makes Apple zero profit. All of those cards require UEFI to boot, while Apple Silicon uses iBoot in its stead.
Then who is going to write the drivers? That's a lot of software development for a miniscule market. Drivers aren't easy, just ask Intel. AMD isn't desperate enough for table scraps, because they aren't hurting financially. Who is going to test the drivers? Software developers need to know well ahead of time, not in a sudden drop. We've had three WWDCs without a test kit, which would likely be in the form of an eGPU, yet nothing but silence. If a fourth year passes, then it becomes even less likely.
It's been obvious that Apple's plan was to replace both the CPU and the GPU inside its entire line of Macs, which includes the Mac Pro. Remember, AMD has only been a reliable source for GPUs in recent times. During the initial planning of Apple Silicon, AMD's offering were as lackluster as Intel's on the CPU side of the equation. Apple has been pushing a unified model for Apple Silicon, and shown no interest in diverging.
If the Mac Pro is a Ferrari, then it's been one that's been neglected for the past decade, as it sits and rots; something that's happened repeatedly. I also think Apple is culturally adverse to handing that control over to another GPU supplier ever again. They've never competed in the bleeding-edge, if they needed to, then they would have made nice with Nvidia already. Apple is going to compare the Apple Silicon Mac Pro to its "best selling model" from 2019, which John Ternus stated was the 16-core Xeon with a W5700X GPU. That's hardly a high barrier to climb.
@GrumpyCoder beat me to the Nvidia solution as I was writing this post. Nvidia and Apple had a nasty divorce, and they ain't getting back together. The Apple Silicon Mac Pro will likely have slots, there are plenty of cards that already work with Apple Silicon, just look at
Sonnet's xMac. However, the notion of display GPUs is dubious.
I realize the riddle of the Mac Pro has been a curiosity for a long time on MacRumors. However, I think when the actual product is released we're all going to collectively say "that's it?". Until then, speculation runs rampant as Apple stays silent, which I think gets them into trouble with professionals. I've heard every theory combined into a homunculus of absurdity, which shall continue until Apple makes an official release. For those folks expecting something different than the other Apple Silicon Macs, I wish them luck dealing with their disappointment. The Mac Pro isn't special just because it costs more. Besides, given the history of neglect, I think Apple is one bad meal away from canceling it.