Given Apple's current locked-down trajectory with the Mac, the Mac Pro was gonna die eventually, and it's for the best that it does given it was reduced to little more than a massively overpriced Mac Studio grafted onto a useless PCIe backplane; a $12k grift, basically.
PCs at least are still modular and expandable; for now.
The cases of old ones are on eBay, as I mention in another comment.
And it looks like someone has made ATX mounting kits.
https://thelaserhive.com/product/mac-pro-atx-kit-with-psu-mount/