I get the upgradability factor - I brought my 2019 Mac Pro from the 8 core to the 28 core by upgrading it myself.
After it was basically "maxed out", the platform itself became the limitation - i.e newer CPUs overtook the Xeons.
I think the 2019 Mac Pro had a pretty good run, and is still excellent. I am happy the 2023 Mac Pro exists for the PCIe slots and being able to use Apple silicon - but it may likely have a similar lifespan as the 2019 for most people. If you find yourself needing more ram or GPU power, then the 2019 remains a much longer lasting computer.
I am curious what the "next" Mac Pro upgrade will be. Just M4 Ultra in the same chassis? I'd imagine they may do that since it less work than a new redesign.
The Mac Pro has a lot of "halo" product style attention on it that I am sure Apple likes to keep alive. Remember how they made special factories in the USA for them, even if they are low selling, it still is an impressive machine.
There is no other PC or Mac in the world with that built quality and engineering IMO, price and value aside. (I happen to think it is a good value for those, like myself, who need it and PCIe)
Heck, as a PC builder, I've seen $600-$1000 cases that are no where near the quality of the Mac Pro chassis alone.