What makes an SST impractical is the economy of scale to build it. There just are not than many routes that would benefit from Mach 2 vs. Mach 0.85. All the routes that could support an SST are trans oceanic. I mean even a flight from LAX to JFK would not be materially shortened by supper sonic flight when block time is considered. Taxi time in an SST is the same as a turboprop. Time in the speed restricted environment of terminal airspace is the same for an SST as a high subsonic jet. So, MAYBE a flight from LAX to JFK would cut an hour off of block time. How much is an airline willing to pay for a brand new (unproven) airframe that will require retraining of mx personnel and an additional box of spares just so they can say, "We get you there a little faster"?

So, only ultra long haul routes can justify the cost for the airline. But look at the A380. No real problems with the design, but it isn't selling. Airbus may lose a ton of money building a fine aircraft that nobody buys.

Kinda reminds me of the L1011.

No company will spend the zillions of dollars to design, build, and test an aircraft that will only sell 50 airframes.