Lots of businesses refuse to open cases to upgrade. They just scrap the old machine (or sell it, or give it to someone less important), and buy a new one. This is businesses, with professional IT staff.
Consumers are mostly even worse.
So the number of people who'll bother opening their cases is very small. The number of people who'll do so, and buy Macs is even smaller.
With thunderbolt, you can do a lot of upgrades just by plugging in a box. This makes upgrading a lot easier for most customers. And since that will be the only upgrade path, there'll be more (and cheaper) components than there are today. Plus, the resale of these components should be higher, since they won't just be sold to geeks.
So I'd say that the new MacPro will be *more* upgradable, to most people, even most professionals. Especially the ones who buy Macs.