I would agree, I think USB would stay the standard since it is backwards compatible with older devices. It would be interesting to see if USB 3 would ever be thought of as an internal interface for Drives.
But TB could be seen as a (to your point) a high end interface or even a replacement for internal drives since it is so fast. not that rotational drives could ever use the speed advantage.
The one draw back I see to SATA is that is is not daisy chain compatible or even Hub compatible. That is where i think PATA had an advantage over SATA but it did alleviate setting jumpers on the drives(drive replacement for dummies you could say)
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So if you could have 1 or 2 TB connectors for internal you could then daisy chain several internal drives instead of multiple SATA connectors
I see TB as best of both worlds speed boost of USB3/SATA3 on steroids with the daisy chain ability of Firewire.