When iPod came out there were numerous successful portable music players already on the market. The iPod skeptics were not skeptical that there was a market for a portable music player--they were skeptical that the iPod's particular combination of features and limitations would do well.
With supersonic passenger service no one has demonstrated that there is actually a viable market for it. The two prior attempts were both heavily subsidized by government (Russia for the Tu-144, France and the UK for the Concorde).
It is quite different to ask "why do you think this product will do well against a bunch of established, viable competitors?" and to ask "why do you think this product can succeed in a market that everyone who has tried before has failed in?".
IIRC Concorde development was highly subsidized by the government but it became a sustainable business for British Airways as long as there were wealthy business travelers willing to pay a huge premium to get between the financial centers of NYC and London in 3 hours. After 9/11 a lot of that business went away and Concorde wasn't viable anymore.
Boom is taking the government's role, not BA's. I once did a back of the envelope calculation which suggested the government could have lost less money paying for a NY- Europe business class ticket on a regular aircraft for every person ever to fly Concorde. It was as spectacularly bad commercially as it was impressive technologically.
It was a sustainable business for BA because they got several aircraft at giveaway prices (the minister who sold them conceded it may have been the worst deal ever negotiated by a government!) which they could charge extortionate rates to fly on, but they still weren't flying more than one of them at a time very often.
