This would probably push some high-end audio professionals away from Logic. One of the niches Mac Pro has been popular is audio production. And with cheesegrader the ability to slot in many-many different audio interfaces into a box instead of dangling out to various PCIe enclosures has been a big win.
Here's a good video how it looks like: https://www.youtube.com/watch?v=kIQINCWMd6I&list=PLi2i2YhL6o... (at 1:40 Neil Parfitt shows Mac audio setup his before and after).
Feels like it'd just create a market for a big rack-mountable multi-bay PCIe enclosure, with its own internal power supply, that you could connect with one ore more thunderbolt cable. I don't see any reason why a solution built around a Mac Studio should have to be significantly more cluttered.
I don't know if such a solution exists right now, but I'm thinking there's a fair chance it will soon as the Mac Pro disappearing creates a demand for something like it.
Thunderbolt is really an unsung hero here. It is surprisingly nice to be able to move various components around my desk that would have otherwise sat in a huge tower hogging all the PCIe slots they can find.
Agreed, I've been doing experiments and it's wild to me what "just works" in a secondhand eGPU case or music production PCIe boxes.
Dual 10G NIC cards, way cheaper than a comparable dongle 36 HDDs in JBOD, absolutely! 12 optical drives, sure!
The Thunderbolt offerings on the current Mac lineup offer dramatically less bandwidth in total if that matters for a given use case. Thunderbolt 5 is the equivalent of PCI-E Gen 4 x4. So if all 4 of the Thunderbolt 5 ports on a Mac Studio can run at full speed, that's still only the equivalent of a single gen 4 x16 slot. That's less than half the bandwidth of a basic consumer x86 CPU, to say nothing of the Xeon that was in the previous Intel Mac Pro or a modern Epyc/Threadripper (Pro).
This is a big reason why things like eGPUs kinda suck. Thunderbolt is fast for external I/O, but it's quite pathetic compared to internal PCI-E.
Reports as pointed out here have shown that x4 to x16 for most GPUs and common loads is a 1% to 10% loss of performance - hardly pathetic. In many (gaming) cases, it would be unnoticeable.
The DAD AX32/AX64 is such a thing.
The video you linked is from 2019. A lot has changed with Thunderbolt capability and the Studios now have enough ports/bandwidth to handle audio processing needs to multiple boxes.