The people who migrate are basically the people in the intersection of technically inclined, "we need social media," and "big tech is bad."
I think having a protocol that can be abstracted away to meet user needs is a great idea. But as it currently exists, Mastodon will always be niche. Too many weird tradeoffs that can only be forgiven by passionate advocates who understand their technical roots. Perhaps there is further development to the protocol or abstraction that can change this, but as it stands, Mastodon is only for passionate social media users who are technically inclined and resentful against big tech.
Most artists I follow also just dropped Twitter entirely and are on Instagram exclusively now (much to my dismay, the Instagram UI is nails on chalkboard to me.)
I've seen numerous anecdotes from more technically-inclined types saying that their Mastodon posts not only get greater engagement than their Twitter posts did, but that the average quality of engagement is much higher and closer to an actual conversation.
So I can see a more intimate setting benefitting them more than a more traditional content creator who is already used to all those hoops.
Reddit, not so much. Not that I don't think a lot won't simply go back, but a lot more people will probably leave, maybe even enough to create a network effect to begin the sparks for a new, proper competitor.