So, if the classic Mac Pro is so great, why is there such a huge second-hand forest of them?
Macs that that had reached the limits of their supported GPU-life being replaced with Windows workstations, pure and simple.
Apple tried the road of a very focussed machine, and "go away if it's not right for you", and people chose "go away". Some went away to other Apple products, a lot didn't. Now they're discounting FCPX for edu customers, I would suggest because every school that would teach video, already has an AdobeCC subscription which includes Premiere. Blackmagic are giving Resolve, which appears to be an increasingly capable NLE, away for free. Capture One which is the most Aperture-like of the PhotoDAM apps, is cross platform. Affinity Photo, which Apple gave a design award to, is going cross platform along with everything else Serif does - the entire content creation industry is in the middle of an escape plan from Apple's products, and that is the dominant narrative.
If you don't want a Box O'slots, Apple makes a computer for you, two of them in fact, and soon to be three - Macbook Pro, iMac and, soon, iMac Pro. A less specialised, less focussed, and more generic-pc machine is the only way to get sufficient versatility in a machine, that it can capture every remaining edge case, which it needs to do in order to provide a return on investment that is competitive with anywhere else in the company the money could go.