I had been wanting this display most of the 2020 calendar year and it was always a long shipping delay with Apple. I finally pulled the trigger when I saw stock at Micro Center. From my experience, retailers like Amazon, Micro Center, B&H, and Adorama seem to get stock fairly regularly, and offer way better pricing.
My best guess is, the display manufacturing is being effected by the global chip shortages and it takes them a long time to fill an order. Or, Apple never expected a high demand for the XDR past the initial run so it was always going to be a built by order product?
I don't anticipate Apple will discontinue the Pro Display XDR at this point in time. They may soon offer a consumer grade display, but it shouldn't hurt XDR's market as this was never marketed to the mass. If rumors are true and the future MacBook Pro may soon get XDR (similar to iPad Pro), over time, this display should in theory become cheaper to manufacture as the technology becomes more standard. If anything, that would push them for a price drop on Pro Display XDR rather than a discontinuation. Apple made a similar move on the once gigantic 2004 30" Apple Cinema Display in late 2005/2006.