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.