You make some good points, but BA received their Concordes for free. It's easy to turn a profit with a 150 million pound gift. Air France did not have it as easy. Boom's customers will not be getting their planes for free. And a 50% efficiency saving does not translate into 75% cheaper tickets. And my experience is that claimed maintenance savings are never as good as promised, and that goes doubly so for the "A" variant of any plane.
Only the last four white-tails were close to free, they cost 1 Franc each.
The original UK Government-borne £115 million contract cost of the BOAC / BA Concordes was being recouped at a rate of 80% of their operating profit until in 1984 BA bought themselves out of the arrangement for £25 million.
They specifically mention that both Airlines didn't have to pay for the planes:
> Part of the reason the airlines could claim some profitability on the Concorde was because they did not have to incur capital costs on the planes. [...] The governments of England and France underwrote the costs of building the 14 Concordes, which were then turned over to the two national airlines.
[1] http://www.nytimes.com/2003/10/24/nyregion/for-concorde-econ...
FWIW Concorde ticket prices were so high because they were expected to be so high. By clients. BA raised their prices significantly in the early 80s to match what customers perceived the prices would be:
> Market research had revealed that many of Concorde's existing business customers had their tickets purchased by their employers, and thought the cost of a journey on Concorde was more expensive than it actually was; thus ticket prices were progressively raised to match these perceptions.
As for maintenance costs, I imagine that one can expect a very substantial decrease due to the use of CAD, better materials and improved practices. Cost reductions of >40% per flight hour were achieved in the military for "equivalent" aircraft between 1970's and 2000's, and reductions of >50% per seat-mile were achieved in the same period for commercial aircraft (eg: early B747 vs B777). Maintenance costs for commercial airlines are a fraction of the fuel costs in any case.
Finally, it is worth remembering that the Concorde was a very profitable business for British Airways, with operating profit of ~40% for the Concorde!