That's not much of a problem since they can just modify the airports like they did for the A380. (Not that airplanes need to keep getting bigger for Airbus/Boeing to keep selling planes.) However, if no one flies in 100 years (transporters, etc), then it is unlikely Boeing or Airbus will still exist.Boeing and Airbus? There are not many companies making airplanes and it seems difficult for new ones to become larger.
Aren't planes practically at their largest size possible because airports cannot take airplanes over a certain wingspan and the largest airplanes today have already reached that maximum?
Technically Airbus isn't a company anyway, it is part of EADS.