If Threads, which has the biggest userbase of any instance, is allowed to connect with Lemmy, their communities will naturally become the most trafficked (embrace).

Over time, the Lemmy userbase will largely move everything to the communities with the most activity. Facebook could also add its own proprietary features that Lemmy users wouldn't be able to see or use without the Lemmy devs somehow found ways to enable compatibility (extend).

Then, after a while, Facebook could simply say, "Eh, ActivityPub isn't worth it," and turn it off, leaving us without most of the communities we've become accustomed to and without most of the users we've come to know through those communities (extinguish).

This is known as "Embrace, Extend, Extinguish".

Embrace a competing product and enable compatibility with the product. This may seem like some sort of goodwill gesture, but it's not. Companies are in it to make a profit, and any users not using their product is profit lost.

Extend the capabilities of your own product beyond that of your competitor's product, creating compatibility issues. Some existing users may jump ship to the "better" product because of this, and new users will be pressed to use the "better" product because of the compatibility issues.

Extinguish the competition by disabling compatibility with your competitor's product after they've lost users and stopped growing since you offer a better product with more features.

By using this method, you may successfully kill any potential competitor before they become a problem, nipping its growth in the bud.

You can find more information and examples on the the Wikipedia article about this method: https://en.wikipedia.org/wiki/Embrace,_extend,_and_extinguish