In a competitive market, it really will be "pay less if it's not urgent". In particular, if you're a peon flying coach, you want some people flying first class, because they are effectively subsidizing your ticket. Bring on the rich!
The ecconmics of airplanes is different and do not apply. Here you want enough people paying for the cheap service that it makes money so they don't kill it. There is no advantage to a class of service that doesn't pay for itself in this space, you just move up to the next one.
Actually the worst thing would be most people choosing this as they may decide that the cheaper class isn't worth serving at all.
I don't think this analysis is correct. No one's going to offer a service if it's cheaper not to, yes. But if there's only one class of service, you have to make $X on each order to cover your fixed costs. If there are multiple classes, the cheapest class could end up being $(0.8X) and still allow you to make more money overall.
Think of it as an SLA. People optimise for different outcomes, some people need their groceries in a 1 hour timeslot because they have other commitments.