The Concorde wound up spending a lot of time at sub-sonic speeds, but really I think if we look to the future of air travel we can imagine surpassing supersonic speeds as well. There are already companies working on hypersonic (Mach 5+) designs with an eye towards consumer applications; obviously rich consumers to start, but that's how it often goes.

Those are some truly societally transformative speeds. You can go from the west coast of the US to Paris in less than 2 hours.

But we can't get to that without iterative improvement.