Just to be clear about this specific case: There is "evidence" (Twitter posts) coming out that showed that all new users of Parler were shadowbanned by default

Just to be clear about this specific case: There is "evidence" (Twitter posts) coming out that showed that all new users of Parler were shadowbanned by default, until their posts/content were approved by a small group of "right-think" moderators. So while your hypothetical isn't invalid in general, it's really not what was happening here with Parler. What they said they were trying to be was not at all what they were actually doing - a common theme in extremist spaces. But your overall concern about the concentration and centralization of our communications is legit. There's no easy answer there.