It might be better to say that they form a system which is economically optimized as a whole. It is genius, for instance, that frequent fliers get upgrades from coach to first class when they're available because it means if a first class flier changes their plans or wants to fly on short notice an economy flier just says in coach and won't feel mad about it -- the first class seats are 100% occupied in terms of providing somebody a somewhat premium experience but are 50% occupied with full revenue customers and that 50% occupation is part of the completely premium experience because it means it is always available.
Yes, I was imprecise in what I wrote. There's a lot of cross-subsidization in various forms going on that makes all-business class at business-class prices tough for airlines even on routes like NYC to London--which have existed but not sure they do at the moment.
I heard only place where business + premium economy works is ultra long routes. Where people are willing to pay the premium of single leg over multi leg. So it seems people might be willing to pay for time, but not for comfort.
