Why people buy the Studio with the high ram config is actually the unified memory. This is unique to Apple. I'm not sure what Mac Pro would do with PCIe cards . It would be useless for AI because what you want is unified memory that can be used by the GPU/AI not just ram.
PCIe cards would indeed be useless for AI unless Apple supports third-party GPUs, but there are certainly some pro creators that would still prefer to have them. I myself work in large-template film/game scoring and while we all love our Mac Studios, they're usually housed in a Sonnet chassis so that we can continue to use PCIe cards. Had Apple kept them in parity with the Studio w/r/t CPU and RAM, the rack-mount version of the Pro would've been a no-brainer.
Its not entirely unique to Apple: the Ryzen AI Max platform (in the e.g. Framework Desktop) is a unified memory platform. The PlayStation 5 also has a unified memory architecture (which given the chiplet was made by AMD, not too surprising) (people sleep on PlayStation hardware engineering; they're far better at skating to where the puck is headed than most hardware tech companies. remember Cell?)