>It would be harder to provide a premium service airline with all more-expensive seats
It's been tried and, by and large, it doesn't work because too many people just buy on price. There are some minor exceptions--e.g. British Airways all-business London City to New York route but that's obviously a pretty special case.
Economy Plus is just more legroom. It may include priority boarding as well. I believe it primarily exists as a perk for frequent flyers (which I am). I've been told that, otherwise, you may not be able to book it until some number of days before your flight.
It's been tried and, by and large, it doesn't work because too many people just buy on price. There are some minor exceptions--e.g. British Airways all-business London City to New York route but that's obviously a pretty special case.
Economy Plus is just more legroom. It may include priority boarding as well. I believe it primarily exists as a perk for frequent flyers (which I am). I've been told that, otherwise, you may not be able to book it until some number of days before your flight.