I don't think it's possible to get "mainstream communities" away from Discord to any open alternative. Not after they got a taste of maximal convenience.
For forums to make a comeback, they'd have to reach the same level of convenience. People would like to have a single account which allows them to browse forums and use them without having to register over and over again. People who want to run a community shouldn't be required to have technical knowledge about how to host a forum software. And of course, things generally shouldn't be janky.
Those are the minimal requirements which are only viable if a single huge provider runs the show.
@volpeon@icy.wyvern.rip