I think any argument for decency in dress code falls apart when you get to ULH flights in Y. How others are dressed will always take a back seat to the other challenges.

Traveling for 24+ hours without sleeping in a proper bed and being crammed into Y cabins like sardines is the real issue. Essentially sharing a metal tube of a bedroom with 100s of pax is never going to be pretty no matter how they're dressed.

I think the key for comfortable Y travel is to avoid ULH flights and red eye flights. Book layovers with time enough to sleep a full night. Being rested, it might be easier to avoid getting annoyed at fellow travelers' apparel .