That's a fair point. With this machine you are committed to a certain performance over a long period before you commit yourself to the next machine. With a modular design, you can change the asset in smaller increments, at the cost of having an upgrade process to manage.
I do make incremental upgrades on my personal desktop though (mostly gaming). The last rebuild was triggered by some failed parts, so I ended up doing a new motherboard and processor and switching to DDR4, but I still have the GPU, screen, SSDs, and some other miscellaneous components. If I'd had an out of warranty failure on an iMac Pro motherboard I wouldn't have been able to salvage those. Or perhaps knowing that giving up the computer means losing the screen along with everything else, I'd have just shelled out a couple of thousand dollars on repairs.
I was a pretty early adopter of 4K screens and I'm still using the same one more than three years later.
I'll also point out that the iMac used to be able to be plugged in as a display (target display mode), but the iMac Pro isn't listed on the compatible models page. For whatever reason, they dropped that feature after the 2014 models and decided not to reintroduce it.
Once you're no longer using it as your primary workstation, that gorgeous 5K panel is going up on Ebay or wherever along with the rest of the computer. If you want to spend your money efficiently, I have a hard time believing that's a better deal than a workstation and monitor from Dell or similar.
http://appleinsider.com/articles/17/06/15/2017-4k-5k-imacs-w...