Who's going to spend millions (or billions) of dollars developing better supersonic technology when one of the most important markets is off-limits?
Militaries, for one. But that doesn't mean supersonic flight is completely off limits to passenger flight. There used to be regular flights across the Atlantic at supersonic speeds operated by Air France and British Airways. The last one was in 2003. Did the Concorde program fail because they couldn't fly over the CONUS?
I am not defending the author's economic arguments, but this sentence is quite dubious.
Who's going to spend millions (or billions) of dollars developing better supersonic technology when one of the most important markets is off-limits?
(Edit: remove superfluous word.)