>What financial incentive?

If the network starts to serve an increasingly disproportionate amount of illegitimate traffic for n customers, then the ISP will have to increase throughput (and costs) to continue serving n customers.

It's tempting to argue that they could just pass the extra costs on to customers, but that obviously makes them less competitive, especially vs an ISP that better manages costs.

Many ISPs have basically no real competition. Don't like your cable company? Your alternative is <5Mbps DSL with a tiny amount of upload, high-latency and low data caps on satellite, and if you're lucky, a wireless connection that's even worse than DSL.

Maybe in some places. But, that says they could raise prices now. And, to the extent that this is true (or not), they are still better off with reduced costs.