If you find me flying, if it's for formal business of course you'll find me in a business shirt with tie, trousers and a suit jacket.
Any other time, no matter which class (although I haven't flown F yet), you'll see me in jeans and a collared shirt, with closed shoes. And I do not mean ripped or torn jeans. The shirt is collared - not a t-shirt - but obviously long or short sleeved as appropriate.
I think I adopt a smart casual style when travelling. Personally I don't see anything wrong with that, even in premium classes. Enforcing people in premium classes to wear formal attire may seem more classy and distinctive, but in the end I think is unnecessary and pompous.