I don't think the advantage for Mastodon is being able to do anything that Twitter can't. Instead, it's being able to do everything that Twitter can without a lot of the negatives that come with Twitter.
My view is that, fundamentally, Twitter is a good platform, but it suffers from a number of toxic elements (dog piles, turning everything into angry politics, new "features" designed to increase engagement/number of ads you see, etc.). I'm guessing that "Twitter, but without everything you hate about Twitter" is enough to overcome the network effects, especially as Twitter keeps finding new ways to annoy its users for money.
But maybe I'm wrong about that. I guess we'll see.