You'd have to completely finish boarding for lower classes and then give a decent grace period for first class passengers to just show up at their leisure.

Also first class (and often business) are kinda separated so no one stares you down.

I fly premium economy sometimes and you're the first to board and sit upfront and then you have to watch everybody walking by!

It's a world of difference between premium economy and business, sigh.

Norwegian Air Shuttle had a pretty cool idea with their premium cabin. It's not crazy expensive but the seats are so much better!