What's changed is the market. It wasn't economical to run the Concorde for most of its service lifespan, because demand for fast business travel was limited and fuel prices were high. Then globalization and income inequality happened. There are a lot of very lucrative fields right now where it's beneficial to be in SF in the morning and Shanghai for dinner, then back to SF for the next morning. And fuel prices are on their way back down.
They can probably build it a bit more efficiently than in the days of the Concorde, Tu-144, and Boeing 2707, but the real driver here is the existence of wealthy firms who will pay a lot to shlep their employees across the globe in a day.
Yeah, I think this is the new driver for SST. There's now a class of customers who can easily afford whatever the ticket price will be, so a market is assured.
Unfortunate for those of us who would prefer not to hear sonic booms day and night.