It seems to be growing pretty fast, and importantly has celebrities and such. It's going to replace Twitter, but Mastodon has staying power that BSky doesn't.
Honestly, I feel like Mastodon is kinda never going to be like Twitter, even if its user count were to grow by two orders of magnitude. There are several reasons why, as the other replies point out, but the most important (IMO) is that Mastodon is just not a profit-driven platform. And if Mastodon is not a profit-driven platform, it is not designed to maximize user engagement. And if it is not designed to maximize user engagement, it is not designed to encourage toxic behavior.
At the risk of sounding like a door-to-door fediverse evangelist: the good thing about mastodon is you can always move to a small and quirky instance, or even start your own small and quirky instance and defederate from any parts that you don't like.