Interesting discussion. I guess I tend to dress up more than most when I fly, but that's because I tend to dress nicely most places I go. I don't do it to impress anybody; I just like to look presentable and I feel good about my self-image when I feel like I look my best. Unless it's the middle of summer, my go-to travel trousers are a pair of wool slacks that are softer than pajamas.
I still think it's tacky to wear gym shorts or sweatpants in public, and flip flops are for the pool/beach. Some days I'm convinced that Americans won't be satisfied until they can wear their underwear in public. Planes get so cold, so I can't understand how bare legs, bare arms, and bare feet can be comfortable.