I hear that, but I see it the other way around.

Like you, I have some niche interests, and to be honest the Lemmyverse is still lacking on that sense. I'm here more in hopes of what it might become someday, than because it appeals to me as it is. Most communities I'd like to participate of simply don't exist here. Yeah I know "why don't you make them", but I don't have the time to cultivate and manage a community. I find myself hopping back to Reddit because as much as that place is on a decline, it has those communities.

Like you say, integration or no integration if the communities and content isn't there people will not come, or they will lose interest. This is already happening. Even with the occasional exodus only a few communities manage to be self-sustaining. Facebook can't kill the Fediverse. Obscurity can kill the Fediverse. Your disaster scenario of what maybe could happen with Facebook is something that is on the verge of happening without Facebook anyway, all that might take is that one of the handful of people keeping the little movement there is in some of the niche communities gets tired and gives up.

As much as Facebook may control their platform and integration, they don't control the people, and I'm sure some amount of people will be like "this place sucks" and hop over because they will see there are alternatives. If Facebook sets it up and then cuts it off, there will be people who will miss it and decide to move to the Fediverse for good. A small fraction of Facebook defectors would be enough to make this place much more lively. And again, would people just give up without Facebook content? It still seems like this is underestimating the people who want to make this place thrive or wish not to be beholden by Facebook.