Over the long run this is bad news especially for Twitter. This is turnkey hosting, all you have to do is work the admin panel. I think we a crossing a threshold here in social networks and breaking silos.
The problem with Mastodon are that the weaknesses of Twitter remain, it is too easy to have pile-ons, mobs etc. And so much depends on how well the Admins of each instance manage these things. Plus there will be a lot of churn.
But with all that said a lot of small groups (including fringe groups) can have their own social network and it’s affordable.