If they become so ubiquitous that all you see are Threads messages, all they have to do is start adding their own extensions to ActivityPub and degrade the experience of everyone who is not using their app.

Embrace, they join the fediverse seemingly in good faith. Bringing their larger userbase to massively increase the size of the fediverse.

Extend, they add some features that are convenient when interacting with their base across the fediverse. But these conveniences require proprietary software integration.

Extinguish, once enough users and platforms are tied into the conveniences of extend, they use that to force compliance. Stricter and stricter rules on their proprietary software. Comply or die.

The fediverse won't be gone afterwards, but if it EEE works then we will end up very stifled.

Here is an example of a corpo dealing a blow to an open source project. The article covers an example of Microsoft and Google killing a competing open source project(s).