As a boomer, I try to be accepting of today's more than just casual attire. But what makes air travel any different from an other situation? Sure my mom wore June Cleaver clothes when she flew in 1960, but she wore the same outfit in any other public situation as that was the norm. I might also add that the norm was also a ticket from Philadelphia to Chicago that cost the equivalent of a month's rent for a lot of people in the highly regulated CAB environment - and maybe that's why it was an elite form of travel that meant gloves and hats for women, suits for men, and even coats and ties for boys and party dresses for girls.
We are much more casual today and air travel, for better or worse, is open to virtually all. Additionally, cruise passengers today don't dress anything like those on the Queen Mary in 1938.
It's a different world.