The armrest thing bothered me for a while, but over the years I've put in many miles on "chicken buses" in 3rd world countries. Recently in Guatemala, it was old school buses, with seats designed for 2 reasonably sized people or 3 petit ones. Custom is that 4-5 are crammed on the seats so no one has to stand. The upshot is that the people closest to the aisle may actually not even have rear to seat, but the sardine factor is so high that they can sit even so. Very close contact, shoulder to ankle.

Somehow shared armrests seem desirable!

Gail In Eugene