Comment Won't go away, just stay niche. (Score 4, Informative) 355
USB is the "mainstream, use for anything" connector. USB SS+ with type-C and 100 W power delivery makes it even moreso.
Thunderbolt is external PCI Express. Over long distances with optical cabling. Yes, there are few places in which TB is better than USB SS+, but in those places, USB SS+ can't compete - at all.
Need a 20 Gb/s connection to your storage array in the next room over? USB SS+ can't do that. Need an effectively-zero-latency connection to an external sound/video editing rig? Yeah, PCIe is your format, over Thunderbolt.
And don't expect Thunderbolt to sit still, either. While USB has plans to increase speed, so does TB. TB has PCIe3 coming up, and other improvements.
No, I never expect Thunderbolt to become even as mainstream as FireWire was, but it most certainly won't just go away, either.