There are Thunderbolt to PCIe boxes that you can get and put a PCIe card in it. I have one made by Atto for Dual 10Gbps ethernet. Its basically a box with a PCIe slot in it that uses Thunderbolt. In many ways, its no different than pro users today who use MacBook Pros. This is just craploads faster.
But thunderbolt speeds are expressed in Mbps while PCI-E speeds are expressed in MBps. So there is a whole magnitude of speed difference. Now if the speeds were actually comparable then I would say this design is genius. Because it would be sized for most people and those who need expansion would attach a box. But given the speed difference it isn't quite there yet.
So as I understand it thunderbolt 1 was PCI-E 2.0 x4. So this is PCI-E 3.0 x4. Now perhaps most of these speciality expansion cards can run at that speed effectively. But somehow I doubt it if they are to accelerate 4k+ full data rate raw video. Plus Thunderbolt has some overhead.
If someone can get the job done on MacBook Pro then this would be great. But this is supposed to be an upgrade for current Mac Pro users who have likely upgraded their machines. It's a whole other level of performance expectation.
To tell you the truth I don't really know much about what video editors need but I'd imagine they need everything they can get. At least some of them. And those people will be pushed to Windows. So this machine by its design is pushing away a subset, however small, of its prior users.
Also while I know their revenue model depends on people buying whole new machines it's disheartening to know that not even one (overpriced) option exists for people who want to upgrade or expand a Mac OS running machine. The old Mac Pro premium pretty much priced an upgradability tax. And people accepted that. But now that's not even an option anywhere in the line up.