Thunderbolt is not a replacement for a mac pro. It can only handle today's medium range graphics cards. What happens if you want graphics and extra storage and something else? You are stuffed. That's what.
You're wrong.
If you want more than 5 internal harddrives in a MP you are screwed too. Get a thunderbolt enclosure that can hold just as many hard drives. Daisy chain them and bam, no need for internal storage because you have thunderbolt that provides the same speed. Whats the benefit of internal storage anyways? A thunderbolt enclosure would stay in the same place much like a Mac Pro. And as of right now it can only handle medium range GPUs. But consider that thunderbolt is a baby right now and is still growing.