They don't offer the 512 gig RAM variant anymore. Outside of social media influencers and the occasional AI researcher, the market for $10K desktops is vanishingly small.
Pretty sure the M5 Ultra will be out after WWDC, so my M3 Ultra is (while still completely capable of fulfilling my needs) looking a bit long in the tooth. If I can get a good price for it now, I might be able to offset most of the M5 post WWDC...