That one big hurdle is "What's the fucking point"? The number of people who need to get halfway around the world six hours faster and who are willing to pay ten times as much for the privilege is vanishingly small, and frankly, that's not a problem that needs solving.
I mean Boeing can't even build subsonic airplanes any more and Airbus. . .well, they have "Bus" in their name. Who needs a supersonic bus?