Users in general aren't sophisticated enough to do this themselves. ISPs will offer it for a price and users will pay more to have ad free internet while simultaneously benefiting ISPs with bandwidth reductions. Some ads will be allowed through of course, but with significant costs associated with the advertising, the volume will be reduced significantly, like the difference between spam email volume vs that of postal junk mail.

If big sites like Facebook decide to fight it in a cat and mouse game, ISPs can hit them with advertising fees directly or throttle their traffic in retaliation for cheating the system. If browsers like FireFox try to defeat it by doing DNS over HTTPS, then ISPs can funnel 1.1.1.1 directly into the trash. AT&T did this and said it was an accident. I'm sure it was more like a test. They are aware of what underhanded scheme FireFox is up to. Mozilla isn't fooling anyone with their "security" cover story.