The real reason why people are "migrating" to Mastodon right now (and can't seem to stop talking about it on Twitter) is one and one only: to signal their tribal affiliation. Twitter used to be controlled by the blue tribe and now it was bought by a member of the red tribe. That is all that is happening. Nothing more, nothing less.
I don't know if Elon Musk will destroy Twitter or if it won't matter in the long term. I suspect the latter, but I'm close to 50/50 on that. What I am sure is that the average Twitter user will not actually stay and use Mastodon right now. Most people are on Twitter for the audience and/or to follow those with large audiences, and Mastodon does not have that not will it in the near future (which makes it sound like a really nice place to hang out now that I mention it :)
I would also point out that, in my experience, internet communities are great until they get noticed by the mainstream. Once they become mainstream, they attract huge masses of users that want to fight for their political tribe with high hostility / low creativity content, and the eye of Sauron (politicians in general) falls upon it, and opinion piece writers start fanning the flames of moral panics, and the good times are over.