The new Mac Pro tower seems to have been designed with commercial video production and scientific computation in mind, and I suspect it's doing well in its target markets, where neither cost nor bulk is a concern. But it doesn't seem like a platform, as Apple used to say, "for the rest of us."
I'd really like to see Apple do a refresh of the 2013 Mac Pro, which has an excellent form-factor for still photography, but I don't see that happening and for now I'll stick with mine (I've upgraded the primary memory and mass storage with aftermarket parts). It's performance with Lightroom and Photoshop is still satisfactory, and it also does okay with the few other photography applications I regularly use. At least on paper, though, the new Mac Mini does look like a plausible alternative as long as you're not using software that relies on a graphics subsystem to accelerate computationally intensive special-purpose processing.
I'd also reluctantly consider a machine running MS-Windows as my primary photography desktop if I didn't have so much home-grown software with UNIX dependencies to automate the administration of my various UNIX and Linux systems, which needs to interact cleanly with the computer in front of me.
