If they had just stuck with their original plan launching the iMac Pro as the MP replacement everybody wanting a new MP would go on a wild rampage don´t you think?
No, even w/o the mMP pre-announcement the iMP by itself would have been enough at least to hold FCP.X & ProTools rampage, the problem is Apple two years ago didnt foresee to need vr/ar/ai APPs in the iOS ecosystem, and those Apps requires for devlopers powerful GPUs, add this AMD delays on ZEN/POLARIS/VEGA development put the nails on the tcMP coffin.
The problem with Apple is they only foresee the things they invent, they use to dismiss market development until is too late, VR/AR/ML was almost an ambush to Apple's plan.
OK, that's thinking positive about Apple's leadership, I think they simple planned a spin on the tcMP to AMD, and delays on Zen/Vega put the tcMP update in limbo, then ML/AR/VR requirements where too much for the tcMP restricted TDP, an updated TCMP could only appeal to youtubers (with latest AMD drivers that allow both GPU to tandem as a single logic GPU) and Logic Pros, the same thing as the iMac, the problem still on ML/VR/AR simple, Apple still has nothing descent for ML/AR/VR workflows, while I think ML could be solved with external rigs (tensor processors managed thru TB3 connection), this is a single use case, and the work to adap the ML tools to this very optimal workflow is beyond Apple reach (even Google which is involven on this is on long way to have an integrated IDE working with external TPU rigs), with the time TPU rigs will dominate is the next step following GPUs in ML, but this do not cover AR/VR, even 8K video (I consider 8K video will never go mainstream and hold only as Master Copy format).
ALL THIS FAILURES HAS A TWO COMMON FACTOR: LEADERSHIP AND OVERSIGHT. (include the issues with MBP keyboard, SSD, and macOS naive bugs)
As for a new Mac Pro, the biggest hurdle would still be the GPU.
Exactly, the tcMP with updated CPUs (180W) is very short in TDP even for RX570 GPUs, the only way to Rise the tcMP form factor is with upcoming AMD PRO- APUs (16 zen cores + 32 vega/vega20 cores) which is aimed at HPC market, since this APU could be deployed as a 2P solutionn the tcMP could still hold DIMMs in the central board but GPU will be replaced by 2 APUs, it will deliver an awesome powerful and efficient tcMP, but I doubt Apple has this on sight (unless they plan to launch Two different Mac Pros, one mini MP (tcMP2) and a cheegrated revival with dual socket, multi gpu etc)
I bet Apple simple will release a MP with enough flexibility to host yearly updates w/o require more attention from R&D, likely having everithing propietary, maybe releasing the GPU interface as openSTD as much, so people could swap with non-apple parts with minimal issues, but I have no big hopes to see std PCIe slots for GPUs.