> You can't simple ignore fixed overhead costs (including capacity to avoid congestion) just because they aren't marginal costs.

There is no ignoring them, they're just small relative to an ISP's other costs.

It costs millions of dollars to lay and maintain cable. You have to dig up the road. The equipment gets damaged by weather and bad drivers and idiots with backhoes and has to be repaired on a regular basis. You have to staff a call center to provide customer support, pay for all the advertising these companies do, bill customers, process payments, handle delinquent debts, hire a bunch of lawyers, pay all your technical staff to maintain the network, pay rent and utilities, etc. etc.

That's what your monthly bill pays for. Not one of those things costs a penny less if you can convince people to transfer less data. The only thing that changes is that the ISP has to upgrade a switch to one with more or faster ports, which is a fixed cost of tens of dollars per customer per upgrade cycle, and in many cases it happens regardless of usage because the existing equipment has reached end of life or newer equipment uses less power. The added cost is a tiny fraction of the bill. Make it twice as big and it's still negligible.