I can’t comment on the “critical mass”, since I haven’t got the numbers. But what exactly does “accessibility” mean in this context? What are the challenges of opening an account in mastodon.social?
The first challenge most people have when starting AP / Mastodon is that they are presented with a choice "pick a server" before anything else. That's what I hear most often anyway.
The other challenge that AP has as an ecosystem is that they have been hostile to anyone wanting to build an index or business. People need to eat and they turned off a lot of developers who'd love to make their living building social media tech outside of the corporate oligarchy.
ATProto welcomes all, even if there is the occasional drama or hostility.
It's the same challenge as picking an email server, which is why no one does that.
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Seriously, joining Mastodon is not particularly difficult; people just freaked out a few years ago at being asked to pick a server to join. The joinmastodon.org website has gotten a lot better at explaining what that means and just directing people to mastodon.social if they don't want to pick something more specific, but the "oooh, this extra step makes Mastodon super super scary, if you pick wrong YOU ARE DAMNED FOREVERRRRRRRR" vibe persists.