However, I think that in our case such a scenario is highly unlikely. First of all, the order of events is reversed--ActivityPub and Mastodon have been built after Twitter, as improvements over Twitter, and the current, not insignificant userbase has specifically left Twitter over bad features, management, or misaligned incentives, so there is essentially no risk of losing that userbase back to Twitter even if it joined the space.
A lot of what makes Mastodon's model attractive depends on smaller nodes being better, so it would not have an edge in attracting new users either. So, I think we only have something to gain.