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.

It would make non-Twitter social media providers like Mastodon, Pleroma, Pixelfed and others more competetive, since they offer a better experience not motivated by corporate interests and can explore more sustainable and ethical business models. Twitter adopting ActivityPub would also be incredible validation of everything we've been working towards and lead to more platforms joining the decentralized social web and more exciting and innovative projects being created in this space.