Why isn't it considered viable? The world is 2x richer than the last time the Concorde flew...
One question is whether you can get noise to the level where you could go supersonic over land.
The bigger question is the economic one. Can you operate profitably at price levels that people would be willing to buy sufficient tickets. You're talking about a plane that is basically designed for only first class flyers to use because subsonic is always going to be cheaper. So how many people will fly at a probable premium over current lay-flat business/first for a faster ride in likely less comfortable seating?
The major pressure has been to make aviation cheaper, not faster.