[*]The Cheese Grater Mac Pro isn't really that upgradable anyhow. I've put cards in it (from a very limited set of choices) and mostly had grief. I know many people have had good luck here, but consider that two new graphics cards got introduced this year and people rejoiced, compared to the 20-30 choices you have on Windows. Was the Mac Pro ever really internally upgradable? Other than RAM, disk and drives ... not really. Now we still can upgrade those three, and everything except video cards can go out on TB.
i believe there are three third party manufactures that make cards designed for the Mac Pro. As well as Nvidia driver support for a few dozen of their cards. Apple driver support for both Nvidia & AMD cards. Plug & play support for PC cards have been out for a few years now.
Instead of buying a cheap low end computer and spending more money overall over time, why not just get the high end one I want and not upgrade? So, pick the cards, disks and RAM I want at purchase time.
People who use workstations tend to use them longer then the average consumer, so upgrading them should be the norm. These people who use these to make a living don't want the latest and greatest only to find it does not support their workflow yet.
You know when I go on vacation I always worry about my Pro next to my desk. The rest of my stuff gets locked up, but the pro is too big. So this tiny case, well, I can easily lock it up now. Or even take it with me.
Desktops are not typically designed to be portable or moved around a lot. While this new Mac Pro would be ideal for this purpose, constantly hooking up all those wires, ( Since not much can be upgraded internally ) for all the external enclosures your going to need.
Desktops haven't changed in decades, time for that to change really
I don't believe in change unless it solves particular problems and right now it will cause just as many problems then it solves.