With wifi systems, there are really two different problems, because of the two major choke-points:
1. The speeds that available technology let you wring out of the slices of RF spectrum you are allowed.
2. The speed of whatever internet connection(s) you've purchased to connect the thing to.
Problem 1 is the really fundamentally nasty one. Physics gives you some hard limits, silicon vendors give you some rather tighter soft limits(but at least they raise them every few years) and whiny TV broadcasters and cellular telcos keep you from expanding your slices of spectrum.
Problem 2, unless you are really in the sticks, is much more amenable to pricing-based solutions: it isn't horribly difficult to throttle bandwidth per-device, or do captive-portal authentication, so you can make fairly granular decisions about how much of your cake you want to have, and how much you want to eat. Have you determined that some amount of 'free' internet access is good for local business/a human right/a public convenience that local taxpayers want, just like having the grass mowed at the local park/whatever? Ok, provide unauthenticated access to that amount of bandwidth per device. Do you find that some users of your free service would prefer to use it much more heavily(to the exclusion of a home ISP, say, rather than just at the coffee shop or in the park)? Sounds like you need an authenticated non-free tier that charges more in order to buy more bandwidth to provide to paying customers.
If you are over-subscribed at the RF level, you are pretty much doomed, at least until better silicon or more spectrum become available; but over-subscription at the ISP pipe level is much more fundamentally solvable.