This is a great point. It all depends on how you define “success”.

There is also the tendency for the mainstream successes to suck all the life out of niches. It’s not inevitable, and becoming a mainstream success can itself remove what was valuable to you in the niche.

But if I want to use Mastodon, I’d rather all the people I want to follow are on there.

I'm not talking about success. I just stated that Mastodon is not going to be a replacement for twitter in the sense that it is never going to be a platform with hundreds of millions of active daily users, which is what (for some reason) a lot of people think Mastodon has the potential for.