Basically my understanding of the fear is this:

Meta joins.

Meta makes friends with everyone because they are playing the long con. They act nice to start, play by the rules, etc. nobody thinks they are nefarious.

Meta develops features that users want on top of the existing framework. These features are things everyone actually wants, not what meta wants because meta is hooking people -give them what they want so they use it, then walk it back later, perhaps silently as a “bug” that just never gets “fixed”- perhaps they suppress aspects of the platform, like make the community/individual finding hide the instance it’s on to make it feel more like centralized social media, perhaps they release entirely new features that enhance the experience. Either way, they make small changes because they can, which lowers the experience of those who interact with it because it’s intentionally buggy with FOSS versions. They likely have FOSS versions running so they can test compatibility to make sure they provide a slightly better experience (why wouldn’t they?)

Users join the meta instance for the enhanced features because they don’t actually care about how the fediverse works, they want a seamless experience (which currently the fediverse does not really offer, none of us can say otherwise)

Meta becomes the biggest instances of activity pub, but with enhancements that make other instances super buggy.

Nobody wants to be on the buggy instances so they switch to meta if they don’t care about privacy.

Meta pulls plug. Fediverse dies (they hope) or at least goes back to only being used by the small subset of the population that would never consider the giants to be options. They pulled all the people they could from what already existed.

Now whether or not that’s actually an issue for a platform that actively wants to be “counterculture” (which fedi absolutely does) is yet to be seen… but we already know what meta wants, which is money, and FOSS and money aren’t really compatible. Just on principle. So what meta wants and what they are doing are at odds currently, and I think it’s only a matter of time before they show the hand they have to play.

But I think that’s what the “defederate immediately” group is hoping to prevent. We all joined because we are sick of the hostile takeovers (even if the media companies technically have every right to do so with their own product) and to prevent being beholden to one of the major players. If everyone defederates immediately, meta won’t be able to actually embrace, because they aren’t being embraced back. At that point all they can really do is fork the software, but they would probably scrap it. It’s not exactly a great platform for a company that has their own.

Importantly, why else would they adopt it? What value does free and largely unmonotizeable software actually have for meta? If they wanted to for themselves, they could easily build a Twitter clone. They didn’t. They adopted a free open source platform. Why? What value does that hold for them over just making their own, if they don’t feel threatened by the alternative?