I’m not sure why the idea that a machine needs to be upgradable to be “pro” is still out there. IMHO, that’s 1990s thinking. I worked in a video house for nearly 15 years and none of our machines were upgraded internally after purchase. We added all sorts of connectivity (fiber to MAM and such) via thunderbolt, but the internals were comfortably speced when ordered. The cost savings we had in the iMac Pro were ridiculous over a Mac Pro.
That all said, the iMac Pro either needs to be updated or cancelled. It was an excellent machine when released, but it’s too outdated to purchase now.
I agree with much of your post. if you go to upgradeable platforms you are back in the realms of Windows/Wintel where often no two machines are alike, where then you have the problems of configurations, depending on which cards you have, which bluetooth, which ethernet card.
My iMac Pro is a great machine and has earned its keep, so I'm not concerned at it being discontinued, and there is still life in that machine for some while to come, but where I'm really looking forward to the new iMac line up, where I fully expect the performance to outdo the iMac Pro, and where the monitors too are likely to be upgraded and where I will be putting order in and where the older iMacs are just deployed elsewhere in any less demanding work.
Personally I don't think we need a new version of the iMac Pro, but where I suspect if we do get one, it will be using exactly the same specs as any new iMac, so effectively the iMac Pro as a different engineered solution will be no more.
Yes we might see an iMac specified as iMac Pro with different RAM and higher spec GPU and CPU, but within the same architecture of the new iMac range.
At present I suspect there isn't a higher spec GPU/CPU configuration other than those that will no doubt be announced in the new iMac range, so for now iMac Pro name also will be set aside.
There is no point in tooling up for a new iMac Pro, far better to tool up for the new iMac range, and when a higher spec GPU/CPU chip becomes available its no real hardship to then incorporate that into the iMac range, which is after all what has happened with the iMac range for some time.
Saves Apple redesigning a different iMac Pro, saves the internal architecture being redesigned and saves on marketing if the new iMac range is rolled out and then upgraded accordingly as even more sophisticated silicon arrives.
Mac Pro will still be updated, although even there I suspect upgradeability will be more to do with more RAM, and a few slots for additional high spec cards, but hopefully not going down the route of Wintel, where often no two machines are the same, and where similar cards still have major compatibility problems in setting up equipment, compared with plugging a Mac in and it works.