Pro Tools users with three HDX cards would like the rackmount, especially since their I/O breakout boxes are usually rackmount chassis. Add a rackmount RAID & UPS, stuff it all in a little roll-about deskside rack...
I like the idea of USB4 (TB3) / TB4 for expansion, and there are plenty of external breakout boxes that handle audio & video; but if one needs more Audio / Video horsepower, or ultra low latency with an enormous number of instruments / channels / effects / etc., then PCIe cards are needed.
And the Thunderbolt expansion is the issue folks had with the Trashcan Mac Pro. Okay, yeah, there was the lack of TB2 products. And the whole lack of thermal headroom thing...
But I am a fan of the Cube, and for a 'entry-level' workstation, for an Indie Studio workstation, for a dude tracking the local hot band, for the folks making a low budget indie flick, etc.; the Mac Pro Cube with an audio I/O & DSPs TB3/4 box & a video I/O & DSPs TB3/4 box would be more than enough. And the expansion slot in the Mac Pro Cube allows one to increase GPGPU resources.
And all that makes the new Mac Pro Cube just like the old Trashcan Mac Pro; except there are a plethora of established TB3 products out there, and (even with higher core counts) the use of Apple Silicon should avoid the "painted into a thermal corner" issue...
I want an Apple Silicon Mac Pro Cube..! ;^p