With the dropping of the rackmount 2023 (and 2019 previously) there are no direct rackmount Macs. "Want" has to meet reality that Apple isn't offering one.
If there is no Mac Pro placement to be done with an Ultra, then there may not being any > x4 PCI-e lane provisioning done by a Max/Ultra SoC. That die space could be allocated to more cache and/or more memory I/O with the GPU probably needs to higher end edge cases. Unlikely Apple is going to build ' a bridge to no where' if there are no internal slots in any Mac.
"Nobody actually wants" is sweeping generalization. I didn't present it as a solution for everybody. For users that just want a extra drive or two inside the 'container box' , this will work just fine. Yes, there are others that it won't , but skimming off a subset of what is left of Mac Pro market is likely better than none.
The availability isn't necessarily 'spotty' if the 3rd party fills the role. The 'spotty' aspect is Apple drifting in and out of Rip van Winkle mode on the Mac Pro.
