Seems more like the plane just had bad luck as there seemed to be enough rich people willing to part with their cash to cross the atlantic really quick.
A lot of the costs problems were directly related to the "over-land sonic boom ban" (on both sides of the Atlantic). The aircraft is VERY inefficient in subsonic flight, the bans had a triple effect:
- Longer flights: Meaning higher staff costs, more wear & tear, suppressing demand (no more "breakfast in London, dinner in Washington DC" ads).
- Reduced destinations: Meaning a smaller fleet, making each aircraft suffer from lack of economies of scale.
- More subsonic flight time meaning higher total flight fuel costs
If sonic booms didn't exist suddenly, I think Concorde could have been a success. The real mystery is why the sonic boom problem wasn't found during testing or development? It isn't like they weren't known about at the time...
There was ban that was quickly overturned. The ban was not for "reasons," the issue was noise pollution and it is a legitimate issue. A lot of working is going into NASA's X-59 project[0] to make overland commercial SST more practical.
It wasn't as popular as envisaged for other reasons too: it turns out that people aren't as willing to compromise on comfort for the faster flight time anyway, people who could afford the luxury were more likely to spend the extra on comfort rather than saving time. This kept prices high because there was not enough demand to expand the service which might have dragged prices lower, rounding off a vicious economic circle. Prices were also inflated by the fuel requirements per passenger flight hour, both in general and because the craft was not fuel efficient at sub-sonic speeds.
Furthermore: at least one of those accidents was due to the craft's design and therefore its safety – the one out of Paris in 2000. Though the flaw that caused that to be possible (part of the fuel tank being exposed while the landing gear was down, allowing debris, in this case parts from the planes own front tire in turn damaged by hitting other debris on the runway, to compromise it) was later mitigated that was a real killer for passenger confidence on an already struggling service.
Having said that about the accident, IIRC Concorde does have one of the best safety records of any aircraft overall when considered against total passenger flight hours, much like flying in general is statistically safer than other forms of transport despite individual incidents tending to be worse.
I think a large part of that is passenger demand but also advancements in engine efficiencies. Making the fuel consumption per passenger not horrible.