The Fediverse missed the boat of becoming the next public square because it was never sailing that route to begin with, if you'll pardon the slightly twisted metaphor.
ActivityPub, and by extension Mastodon, are not community spaces; they are tools for building community spaces. There are at least three major spaces currently: the heavily-moderated, mostly lefty area, the barely-moderated, heavily alt-right area, and the area dominated by Pawoo and defined primarily by Japan's legal jurisdiction regarding image media. There are some interfaces between these broad areas, and some sub-divisions within them, as well as some smaller, mutually allowlisted islands that don't federate outside at all.
Building a new public square may be Eugen Rochko's goal, but it's not what most instance admins and moderators want. Joining the Fediverse looking for a new Twitter is and always has been a mistake.