It isn't Netflix's job to minimize their bandwidth usage for the sake of the ISPs. If I pay for 500GB/month at 20Mb down, that's what I should get. If Netflix does decide to increase the bitrate of their videos, than I'm the one that will have to deal with potential buffering issues and/or going over my cap.

As others have mentioned, the ISPs woefully miscalculated the growth of the internet and consumer bandwidth usage and as is the case with capitalism, those mistakes should cost them.