If there is one theme that repeats on these forums it's the deep frustration of pros/power users that Apple has left us out in the cold. Whatever the reasons are, many of just can't imagine why a company with an established user base and plenty of resources wouldn't keep building proper workstations - even if it represented a small fraction of their sales - as long as it turned a profit.
I am of the opinion the nMP was a money loser, which is why it has not received any attention.
Most businesses, even if they have flashy new product lines, don't abandon a market segment they've spent years acquiring unless they lack the organizational wherewithal to do both simultaneously.
Which is the case with Apple. Their organizational structure does not divide products into separate and disparate categories so there is not a separate Mac (much less Mac Workstation) team that drives development. The entire company focuses on one thing at a time (in a meta, if not actual, sense) so if they really focused on the Mac, they would not be really focussing on iOS. And iOS is too important to them to not really focus on it all the time.
Assuming a $500 billion corporation could easily keep more than one ball in the air at a time, why gift the workstation market to other players? It's one thing to say that there is no point slugging it out with Dell/HP/Lenovo/etc in the low-mid range desktop market with it's razor thin margins. OTOH, workstation performance with an elegant OS/GUI (U/X) that appeals to artists/media pros/etc does have a viable niche. What gives?
Again, I don't think that market is very lucrative to Apple, even at the margins they command for it. The most popular Mac for years has been the MacBook Air because it's been the lowest-priced model. It's replaced the Mini as the "entry level" Mac for new users (hence the Mini being ignored, IMO). With the iMac becoming so powerful (and with it's 5K display), it appears to have supplanted the Mac Pro in many workflows, including audio and video production and scientific. The nMP was always an "edge case" and as the MacBook Pro and iMac have become more capable, it's case has become even "edgier".
The thing that's so backwards though is lately Apple has had an enormous interest in producing its own original TV shows. It's ironic, but Apple is going to be forced to hire filmmakers who use Windows or Linux, because of the lack of Mac Pro updates. It's so dumb and ironic. Almost no post houses are using Macs anymore because of Apple's lack of updates. But if they'd just release a new Mac Pro then that would fix all the problems!
Would it? A huge complaint about the current nMP when it was released was no internal expandability and the requirement to purchase two expensive GPUs even if your workflow only needed one (or even none).
It stands to reason a new nMP would be similar, if not identical, in design philosophy. Apple is not going back to the full-tower with lots of drive bays and expansion slots.
MS on the other hand is trying hard to catch back up all the lost ground during the Ballmer days. I don't think they ultimately are too focused on creative pros segment, but as a start it is a nice ground to base on their Surface line up since there is an apparent void left open by Apple not caring, and this crowd was first attracted to Apple in the first place by the seamless marriage between OS and hardware which MS is trying to push now.
Surface sales contracted last quarter so I think it's not so much that it appeals to Mac "professionals" but more the novelty effect of the new form factors attracts a small segment of the "professional" market who move to adopt it early while everyone else ignores it because it doesn't.