Embrace, Extend, Extinguish

1. Embrace:

They implement ActivityPub and connect to all the available servers. A load of new users will see the content here and all the communities here will be absolutely flooded with new content and users.

2. Extend:

ActivityPub will be extended. Many new features will be added, that don't really match the standard, but they are mostly useful so some developers will try to add them to let's say Lemmy. They won't be blr to develop new features on their own and some stuff with threads will always be broken or half baked. Threads users will belittle the users here, some will maybe go there, cause it just works and is otherwise the same.
New users at the same time will most likely go directly to threads cause it's backed by a giant company, always works and has more features.

3. Extinguish:

They will cut the federation.
Communities here will feel empty and most users will just leave. Only the hard core will stay and that won't be sustainable. The fediverse becomes even less attractive for new users and will devolve into a niche community.

Throw 50 fulltime programmers at development and crowd everyone else out. Or fork it with something clearly superior since they can afford to pay people to work on it fulltime.

Right, I've also yet to see what defederating from a hypthetical Threads instance could prevent.