If MacOS Server is being discontinued, are those being orphaned? Or can you run the regular OS headless?
10.6 (Snow Leopard) was the last version where they sold a separate more expensive server edition of the OS.
As for server hardware, I have to imagine the 3rd party 1U Mac Mini rack enclosures are already more popular than Apple's official rack mount Pro. https://www.sonnettech.com/product/rackmacmini.html
Use case for datacenter Macs is basically just as build servers for Mac and iOS projects, anything else there's not much justification for the hardware cost.
In a less segmented environment (i.e. non-professional), having gear in a rack can also stop people playing with it.
https://www.apple.com/shop/buy-mac/mac-pro/rack
https://www.intel.com/content/www/us/en/products/details/pro...
Native development shops running in-house build pipelines I imagine. But even that has completely gone to the cloud in the last 5 years.