Zillions of websites have active discussion boards, despite the success of Reddit and FB Groups.
A handful of webmail providers have centralized a large fraction of all email users. They already have the control you're concerned about.
Non-federated email means you have accounts at several sites, and most sites have membership requirements (customers-only, employees-only, etc). A client app keeps track of all your accounts. A webmail intermediary is unnecessary. The mnm demo gives a sketch of this [1] and the FAQ gives more detail [2].