Doing that would make them money. They would charge the same amount they do now for what they provide now, but remove the data caps. This might cause some people to use the network more and increase congestion, creating a market for a higher-priced plan that puts all your traffic at the front of the line.

It would also both drive and pay for network upgrades, because the people paying the higher price would expect to get the full speed they were sold, so the ISP would have to keep the network at least fast enough to handle all of that traffic. And could afford to because they have more revenue.

Then the better network is faster even for the people paying the lower price at times outside of peak hours.