I imagine air bridgess also facilitate access for the mobility-challenged. Bit difficult/time-taking if there are wheelchair pax and air bridges and it's a long walk up to a 747 (I've done that more than a few times) . At least air stair boarding was once common. Remember that early 737s (mainly, -100 and -200s) used to have (optional) built-in air stairs front and rear and 727s and DC-9s had stairs in the tail cone.