Widespread adoption of DoH would limit ISPs’ ability to both monitor and modify customer queries. It wouldn’t necessarily eliminate this ability, since ISPs could still use these techniques for customers who use the ISP’s own DNS servers. But if customers switched to third-party DNS servers—either from Google or one of its various competitors—then ISPs would no longer have an easy way to tell which sites customers were accessing.

Source: Why big ISPs aren’t happy about Google’s plans for encrypted DNS | Ars Technica