Apple is pointing to whatever runs well and completely in the supported set. They aren't trying to include "happens to work" or "works with these quirks" in the mix. This is a super-expensive display. It isn't meant for "everybody". If you're paying this much money to be a reference monitor, then it should be a "no quirks" reference monitor. Apple is putting this up again reference monitors that have SDI links on the back. How many generic PCs in BestBuy hook up to those? None. They are only trying to sell this to a few people.That is... absurd. No PCs need apply, huh? Or 2013 Mac Pros? Or any computer except the very small list of Thunderbolt 3-equipped Macs running the very latest version of Catalina?
There are a couple of things needed to run the XDR at full capacity.
1. A modern GPU that outputs DisplayPort v1.4 (even a pair of v1.2 ports can't drive this at full 10-bit color).
2. The latest Thunderbolt controller that can pass through DisplayPort v1.4 (need to get ~38Gb/s onto the Thunderbolt network – previous controllers can't).
#2 is what knocks out the iMac Pro (even though meets #1). The vast majority of PCs are on trailing-edge Thunderbolt controllers (because it is cheaper).
The latest BlackMagic eGPUs work because they do use the latest Thunderbolt controller and can pump their modern GPU's DisplayPort output 'downstream' from the eGPU. (Most other eGPUs are actually just enclosures and don't capture the output streams to route anywhere.)
Pro Display XDR is like the previous Apple Display docking stations in that there are no buttons. So if you don't have [software] controls, then Apple's not going to put it on the "supported" list. (It will 'happen to work" with some PCs.)...
Not afraid of that at all. The online store is listing a Feb. 4 - Feb. 10 timeframe for the display, while the Mac Pro arrival times are creeping closer to the present day. Pro Display XDR sales aren't retracting, and Mac Pro sales are. Sales aren't a problem at the moment. (Long-term., it may turn into an issue, but for the next quarter or so it probably won't be.)Was Apple afraid they'd sell too many Pro Displays?
I think Apple is winning in the context where they're up against $9-30K alternatives. When they plow mostly through the part of that group that is in the market for a new monitor, then things may shake out. But if they have taken over 20-30% of that mix, they are probably going to be quite happy. (It isn't like they are selling these at a loss.)