I think Twitter adopting ActivityPub would be a good thing and a victory for the web. Twitter's current lock-in on its users is its strength, and weaking that lock-in by embracing decentralization would remove essentially all network effects-based obstacles to people moving away from it and spreading out in the fediverse.
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.