The way I see your post is two ways: Either the current Mac Pro doesn't really need to be updated or Apple should throw away their other products since they aren't upgradeable, other than RAM.
Another reason I keep my main system running on Windows, it's very upgradeable and I never have any issues. So that may be why an iMac would be perfect for me, as a secondary machine (another reason I bought my PowerMac G4, because it's a Mac but I can't make much money). But still, 2GB for dedicated video will be pretty good for a few years. Heck, not very many systems nowadays don't even come with more than 1GB. Even though my main computer is a laptop it's still just as powerful as a fully maxed out iMac (with slightly more powerful graphics capability) minus being able to go past 16GB of RAM. And it cost less than $1000 so I don't have a problem buying a new one in 5+ years since that's how long I was able to use my previous machine for. It was from 2007.
Again with the upgradeability, I guess my next machine will be a desktop. I just wouldn't need a Mac Pro, a fully maxed out iMac at best. I just don't need a Mac as powerful as the Mac Pro because I can get software that either does the same thing or is the same thing as the Mac variant on Windows. All I really want a powerful Mac for is for Final Cut Pro, Photoshop, iMovie and GarageBand. My PowerMac G4 is just too old and incompatible to be able to do much of that on it.
I'm going to be publicly executed for saying that, aren't I?