I think it's a mistake for them to discontinue Mac Pro, but it's the culmination of a lot of mistakes. Apple never seemed to fully understand all the missteps they made, and after so many years of this they've probably driven away almost all their original tower Mac customer base, so I suppose it was inevitable.
I'm thinking about the progression from iMac Pro to 2019 Mac Pro in particular. Remember how they did a big mea culpa press event admitting the 2013 trashcan was a mistake, and about a year later shipped the 2017 iMac Pro, positioning it as an interim thing they could get out the door while working on the real Mac Pro update? While the 2013 MP was the original sin, that comeback plan also sucked. It challenged people who'd already been waiting for years for Apple to pull its head out of the ground to wait even longer, and those who were patient enough to stick around were presented with an overpriced overdesigned thing that was really lacking in some areas (e.g. out of the box internal storage expansion).
All those customers really wanted was a boxy Mac with a lot of internal expansion, not a halo prestige jewel. If they'd chosen to, I'm certain Apple could've delivered a non-blingy straightforward Xeon-powered tower Mac on the same schedule as the iMac Pro, and then could've iterated on it a couple times before 2019. This would've demonstrated to workstation customers that Apple actually had come around to understanding their needs, and was prioritizing giving them the essentials as soon as possible. Instead they got more confirmation that Apple didn't really understand how to design for them any more.