Earlier quoted context omitted.
The bill explicitly prevents that behavior. "The 'opt-in' nature...would set it apart from other state internet privacy laws... the proposed Maine law also would prohibit any [ISP] from making the sale of customer data part of its mandatory [TOS]. It also could not charge higher fees to customers who refuse to opt in" https://www.pressherald.com/2019/05/29/maine-on-track-to-pas...
They can't charge more for refusing to opt in, so nobody will opt in. The only alternative seems to be a hike in rates for everybody. Which isn't necessarily bad, it's arguably better that people actually know what cost they're paying. It's just naive to assume that this is ISPs getting some "extra money" on the side. This is factored into the revenue from the service. Now they'll have to adjust their models.
One positive is that it changes the criteria of success in the market from "who can provide quality internet at a low price, and mine and sell the most data" to simply "who can provide quality internet at the lowest price".
When new companies enter the internet market (if such a thing still happens?) they will be asking "how can we provide good internet?" instead of "how can we mine more data from our users?"