The iMac Pro is basically a stopgap until the new Mac Pro's arrive next year.

They actually acknowledged they messed up with the 'dustbin' Mac Pro and it's lack of upgradability, which is a first. (https://www.theverge.com/2017/4/4/15175994/apple-mac-pro-fai...). They are working on a completely new Mac Pro to address the issues actual pro's have, but that's going to take a while. The iMac Pro is just here to fill the gap.

I want to be an unmitigated fanboy, but I agree. We need an upgradable package. My PC gaming rig is going on 10 years now, after 2 GPU upgrades. That's a lot of bang-for-buck.