More people isn't always good. But imagine some organizations and journalists that for whatever reason still are on Twitter can now be persuaded to sign up to Threads. Well, now they're in the fediverse.
And if the content from Threads is overwhelmingly bad we can always defederate and the fediverse still isn't destroyed.
The misnomer is in the extinguish part, which is why the meme is pretty weak. Most services do not disappear, they simply stop growing and coast or dwindle.
The same fate is likely for us, due to the superior resources Zuck will have compared to us, once he is connected. We will no longer have any unique value we can provide, there will never be a reason to migrate to us.
However, Zuck had the resources to co-opt the entire development process. He can offer things like video hosting, that are out of reach for most Instance owners. Simply put, he can beat us. He'll have access to everything we have, plus his own resources. Anyone that stands in his way can be bought, he can throw millions without blinking an eye.
Our only real chance is to never sign the deal in the first place. Otherwise our irrelevance is virtually guaranteed. The idea we can actually outperform Meta well enough to pull people from it is frankly laughable to me. Do people get pulled from facebook often? Why not? It's crap, after all.