So if you don't believe what they said about designing themselves into a thermal nightmare, than what do you think the real reason for them not progressing further with the model? Lack of sales? Do you think the new Mac Pro will sale a lot? It is a necessity to some pros and industries, but not at the expense of commoners to purchase.
Responding to criticism in a visible (never mind whether it is sensible or ideal) manner.
They built a big box mac to "show their commitment" to the "pro" platform because they needed to do something big after ignoring the problem for so long (7 years with no upgrades!). Never mind that the new replacement is hilariously overpriced and suitable for maybe 1 percent of 1 percent of all mac users (as opposed to building something with an entry point and price somewhere lower***) - they can now claim they have an expandable box again.
The big box mac that can take generic PCIe devices again enables Apple to be lazy with their own accessory development but not cop flack for it any more.
That was apple's mistake more than anything. Thermal headroom may have been an issue, but that's the case with every single device apple makes. I don't think the current mac pro sales will be any better (i'd wager they'll be WORSE because of the higher priced barrier to entry), but at least apple won't be ridiculed so much for lack of expansion options now, even though they can do exactly the same thing they did last time around by basically abandoning all development for the thing and leaving cards to third parties now.
Like i stated above, they could have gotten WAY better performance in the same or lower thermal and power constraints than the trashcan was dealing with for the past 7 years. Even with new GPUs from 4 years ago (like the polaris based entry level card they try to sell you in an entry level 2020 mac pro, lol). They didn't because they'd be continuing down the path of being entirely responsible for peripherals themselves.
Now they can just kick that responsibility to third parties due to the generic PCIe form factor cards the new box can take.
They designed themselves into a corner with the trash can but it was more about the proprietary form factor required for cards (in a very low volume device) than anything else. Thermal problems have never stopped apple building compromised devices before (and i don't think that will stop them doing so in the future either).
edit:
*** by all means offer the top end machines at the prices they're at. But there is nothing stopping apple building an X299 based platform big box mac in the same case with slots, other than apple's desire to force customers to pay for a Purley based Xeon if they want slots. Or otherwise forcing the customer to buy an all-in-one if they want a GPU that's kinda weak anyway. Super customer hostile... but hey... that's how apple operate in this space.
