I think if one of them goes sour it'll be easier for people migrate to another mastodon instance, and for that instance to grow. When Twitter goes bad, there's not just a convenient alternative exactly-Twitter-but-run-by-different-people around the corner. But those small Mastodon instances could grow if they had an influx (to a point, and probably better so if the influx was gradual).

Edit: especially because federation means that the people who move to the new instance can still see and interact with everyone on the old instance, so they can't be held to the old instance merely by the presence of their friends on that instance. Unless the old instance blocks federation with wherever people start moving to, but still.