Not only are third party GPUs not supported on apple silicon, but thunderbolt has significantly more latency and lower bandwidth than 'real' PCIe implementations, even ones with similarly cut down lanes like oculink.
Apple tried before to push everything out into external PCIe enclosures and people hated it. Maybe this'll go differently this time, the Mac Studio is certainly a much more compelling offering than the trashcan Mac Pro. But I think this is still a shitty and painful situation for a lot of specific users.
Oh yeah, the market for these capabilities is tiny, no doubt. But at least historically, the people that wanted these things tended to be very big lucrative customers, and also tended to be very influential word-of-mouth Apple evangalists.
Just as a random personal example, my uncle was an Apple guy since the 80s, and when I was a kid in the early 2000s he always had a Mac Pro, several Macbook pros, and a bunch of other Apple gear. He played a big role in convincing all of his 3 other brothers that Apple was the way to go, and those brothers then raised kids in households with only Apple computers.
This uncle is still a mostly Apple user, but he's increasingly pissed at Apple, and definitely no longer evangelical in recommending it to people. He needs to have a linux server machine for his home NAS, and another one for some specialized work applications and it really frustrates him that Apple abandoned his market segment.
I think there's a real possibility that if Apple had pissed him off like this in the early 2000s instead of the 2020s, that our wider family might have not ended up as being so Apple centric, so Apple may have missed out on a lot more sales than just a couple of Mac Pros and expensive software licenses.
Apple tried before to push everything out into external PCIe enclosures and people hated it. Maybe this'll go differently this time, the Mac Studio is certainly a much more compelling offering than the trashcan Mac Pro. But I think this is still a shitty and painful situation for a lot of specific users.