As you may have unintentionally alluded too with your reference to Concorde, there is no hard number or line. It's purely a public perception thing. There is

As you may have unintentionally alluded too with your reference to Concorde, there is no hard number or line. It's purely a public perception thing. There is no definable threshold that is "good enough". The tech needs to stick around long enough and proliferate to a large enough cross section of society to become mundane (something Concorde failed to do). The people who are not satisfied will never be satisfied by an improvement in the numbers. What will change their minds is when it becomes clear that nobody else cares and they're just yelling at clouds. Realistically what that means is a generation has to come of age after the tech has proliferated and accept it by default and don't see it as pushing the limits of what's reasonable. From there the acceptance gets back-ported.