I'll refrain from writing the uncharitable version of my reaction to the idea that the Fediverse should be some small, close-knit community forever and instead say that people who want small, close-knit communities based on ActivityPub are free to create them. Mastodon and other major server software supports allowlist-only federation.

People using servers with open federation should expect that their posts will reach an ad-hoc, informally-specified, bug-ridden, slow implementation of half of ActivityPub running on a jailbroken smart light bulb, and that it will behave differently from vanilla Mastodon.