FWIW I think it fairly unlikely Mastodon will supplant Twitter (tho who knows...), but it does seem worth joining:
1) in the spirit of social media pluralism
2) as a hedge against at alarming early signs from Musk on verification and conspiracy theories
3) as, judging by first impressions, Mastodon could becoming a valuable complement to Twitter: smaller but calmer, with particular thematic strengths eg e-democracy, digital rights, economics, law