>CSAM on social media will be 100x suppressed because banning people is way easier on small servers.

No it isn't. Small servers often don't have paid security or moderation, are run in anonymous fashion, and have no profit motive that can even be used to incentivize them against hosting illegal content.

That's visible when it comes to porn. There's a million bootleg porn sites on the internet hosted that show off illegal content. The only site that was ever forced to curate its content was Pornhub, because they're sufficiently large, work in a jurisdiction that has laws and can be held accountable. From a content moderation standpoint going after a million web forums is an absolute pain in the ass compared to going after Facebook.

Which is the first argument any decentralization advocate always brings up (and they're correct to do so), censorship is harder and evasion of law enforcement easier when dealing with a network of independent actors.

What stops Humbert Humbert from joining hundreds of small servers?

You now have 100x the total human effort for mods to review and ban him.