It doesn't need to be mainstream, nor do I want to be mainstream.
I just use it to follow a number of interesting people and to write my thoughts here and there. A bit like how Twitter was in its early days. (Except without the stupid short word limits which encouraged pithy one liners.)
I don't need it to have celebrities or big media organizations. That is literally the worst part of Twitter. I don't need my mom on there, or to get in an argument with the jackass down the street who is angry because underpaid education workers are going on strike. Or to hear random people's uninformed 200 character "hot-takes" on complicated issues.
Further I resent that the tech industry seems to have cultured its own weird industry-celebrity culture of short-thought hit'n'run tech-thoughts in the form of tweets. I have no desire to "show off" my engineering work on twitter in that form or follow people who do so. But I've seen some nice content on Mastodon of people doing neat things and talking about it; the 500 word limit is far better for this.
Twitter is/was a toxic cesspool of narcissism and engagement. I had no desire to have that in my life and didn't really use it...
But I am enjoying checking Mastodon a few times a day and engaging there. A few thousand people leaving Twitter and contributing to that is good. I don't expect them all to stay, but hopefully the good ones do.