My solution is for instances to stop being negligent. Mastodon still directs everyone to create an account on mastodon.social using dark patterns (see https://joinmastodon.org/), which has lead to the flagship instance being far bigger than its moderation team can handle, leading to a situation where it's a major source of abuse and where defederation is too costly for many to consider.

"People will just accumulate on 2 or 3 big providers" is far from an inevitable circumstance, but there are conditions that make it more likely. That, too, is largely down to negligence or malice (but less so than the abusive communications problem).