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.
https://en.wikipedia.org/wiki/Embrace,_extend,_and_extinguis...
So every once in a while a newcomer like TikTok could blow up and become a fad, but some old grandma could just keep plugging away on their hilariously old-school service and still follow their granddaughter's high school swim team hosted on the hip new shiny service all the kids are going for today.
Ah it's one of those pipe dreams that runs like molasses and can't be explained to an average person before they die of boredom.
Except instead of mail, it's microblogging, full-length blogging, image sharing, link-aggregation, etc. that is done with different services that can communicate with each other.
With improved onboarding it could become just a normal thing.
ActivityPub is a way to share social media.
Mastodon is like if Twitter gave you email addresses.
Mastodon is where I can talk with cool people and make new friends :)