The Mac Pro is barely a business for Apple. There's almost nothing about that line of business that makes any difference to Apple. Frankly, the entire macOS part of the business doesn't really matter much to Apple from a financial point of view.
I think they should give people what they want - a high powered upgradable machine. If they don't want to make it upgradable, then they should at least refresh it annually.
That seems to be Apple's thinking but I think this article shows the reason that that's a flawed approach.
Having content producers using macOS is a huge benefit to their brand and I think it's a big part of driving sales of their iOS devices. At the very least, if they lose the content producer market there will be significantly more friction to producing iOS apps.
Yes, there's a halo effect, but I don't think it's all that huge especially with respect to video pros and I bet it mostly works in the other direction - people like their iPhone so they are more likely to buy a macOS laptop or desktop. Nobody is buying a Mac Pro because they like their iPhone.
The driver for iOS development is profit and as long as iOS users are willing to pay more for apps, it will continue to dominate. And if you want to make an iOS app, you are going to buy a Mac, but probably not a Mac Pro.
The Mac Pro is like the iPod in that it exists in this weird space where it continues to be sold but is clearly not something that Apple thinks is important.
I think they should give people what they want - a high powered upgradable machine. If they don't want to make it upgradable, then they should at least refresh it annually.