Re Mastodon, the initial hurdle of having to choose a server is a significant difference to other social media platforms.
Perhaps, but it's no different from email in that respect, except vastly easier to migrate later.
Seriously, if you're not inclined to skim
TEH RULEZ of a few instances to see if their character limits, moderation policy, etc. suits you, you can just sign up for one of the big instances like mastodon.social and move to something better if/when it catches your eye. It's no more of a hurdle than creating a gmail account.
If you're reading this, you've probably already internalised concepts like reading, writing, cursors, double-clicking, usernames, domain names, that weird
https:// thing, passwords, etc. There's nothing magically difficult about 'instances' as a concept (which is the only really 'new' thing about Mastodon, if you're young enough not to remember the pre-2000s internet) compared to the stuff we all had to learn to get here. Otherwise it's just the usual learning what the names for things are.
And I've not yet seen enough of an incentive in being part of Mastodon to consider it worth bothering. YMMV.
That bin fire of bots and ads you mentioned? It isn't one. By design. YMMV.
Critical mass. There's not much point to a social media that consists of you and a weird bloke called Darren who collects novelty underpants.
Again it's one of those YMMV things. There's not much point to a social media that has less than 1000 active users, who ostensibly spend their time discussing a niche mode of transport...