In general I don't think that Mastodon will ever replace or outpace Twitter. Instead, it gives an opportunity for more user-curated experiences for building federated timelines of relevant content with less random discovery. As a tool for academics, artists, etc it seems very useful. I think it will work well as a supplement to Twitter. Users connecting on multiple fronts and establishing webs of like-minded content generation and sharing with the goal of building a network.