The "Pro Market" is alive and well, Apple just have no idea or care about servicing it, it would appear.
And to those who say that this is too many lines for the Mac division to support, my response is that the Mac division's 2018 revenues were $25.5B. Thus, by itself, it would rank at 120 on the Fortune 500 list -- somewhat larger than Eli Lilly, and somewhat smaller than Raytheon and Northrop Grumman. And to say that companies of that size lack the resources to be able to maintain a few computer lines (if that was all they had to do, and nothing else), doesn't seem reasonable. [It's also worth noting that, in 2018, revenues from Mac sales were a bit larger than those from iPad sales, though the iPad may overtake the Mac in the future.]
I think the issue isn't so much that it's too many lines for the Mac division to support, but that the "pro" line might simply be too small (relative to the rest of the Mac user base) for Apple to support exclusively at the current scale at which they operate.
Let's go back and look at an interview they had with Techcrunch in 2017.
https://techcrunch.com/2017/04/04/apple-pushes-the-reset-button-on-the-mac-pro/
At the time, their Mac user base was about 100 million. By Apple's own admission, 80% of Macs shipped are laptops, with desktops comprising the remaining 20%.
15% of Mac users use "pro" apps in some professional capacity, and the vast majority use Macbooks. Single digit (I am guessing 2-4%) use the Mac Pro, and I am expecting that number to be a little less now as I theorise that some would have migrated to the iMac Pro. It's also very hard to define what a pro Mac is because different professionals have differing needs.
So this, amongst other things, makes it awfully apparent that the market for a mid-tier headless Mac catered to self-styled "professionals" is actually way smaller than the online chatter (both here and elsewhere) would have you believe.
That's why the only Mac Pro which makes sense is a high-end market option. It's going to be expensive either way, so may as well market it to those who are able and willing to pay.
I suspect that's why Apple is trying to have their cake and eat it too, by designing their Macs such that it can appeal to both professional and non-pro consumers alike. And if push comes to shove, Apple seems like it will tend to favour the mass consumer over the professional every time.