Concorde was very advanced for its time, but it still had things like vacuum tube technology in the avionics bays etc. that would have been supremely expensive

Concorde was very advanced for its time, but it still had things like vacuum tube technology in the avionics bays etc. that would have been supremely expensive to upgrade/replace/repair. Also construction material etc. were good, but have advanced incredibly in the past few decades. Engine technology too. But there are still physical limitations around size of a supersonic capable airframe and the like, which limits the design of new aircraft and will still keep the price per seat quite high. However, by far, the biggest limitation that I can foresee is that the aircraft will have to fly non standard routes in order to avoid creating sonic booms over densely populated areas.