LOL usb. 500 Mbps = 63 MB per second in a perfect world. Probably closer to 50 MB per second. Needs Thunderbolt.
Well you're actually an order of magnitude off. It's 5Gbps, or 5000 Mbps. Furthermore, USB 3.1 doubles the rate to 10 Gbps, and reduces overhead. With that being said, it's still far from ideal for graphics. Even the latest Thunderbolt standard is only 20 Gbps aggregate. Terrible.
Then you're got talk of optical Thunderbolt connections (it should have been optical from the start, codename Light Peak). But from what I've heard they're not supplying power on first gen optical designs. That means everything you connect to it has to be self-powered. How hard is it to incorporate a couple of copper wires and a couple of pins into the same cable? You're still replacing the vast bulk of the copper, and you make it into an actual USB competitor.
Anyway, for external graphics what we need is a freaking purpose-designed external PCIe standard interface. Not thunderbolt, not other random proprietary connectors. You can integrate USB with it, or leave it seperate. Two cables is no big deal.