I think that it mostly boils down to what most of the users see as the "Fediverse vision". For me, what's most important is that you have communities and users that are providing content to each other selflessly and with good intentions, instead of them being a product that is heavily monetized and manipulated through the content they are served. The Fediverse helps in this regard that it alleviates the problem with such communities - that they come and go, servers die or people hosting them move on. By being part of a fediverse, you distribute this between multiple such volunteers, so a server disappearing isn't that much of an issue.

However, letting Meta in goes directly against this vision, and makes it even worse by letting Meta feed on and monetize content of every user on the platform. That goes directly against what I see as a main advantage of the Fediverse, and it will probably make me reconsider whether I want to participate. But that's only my, pretty selfish and gatekeepin opinion. But my dislike for corporations such as Meta is deep enough, that I simply don't want to have anything to do with them.

But other users may have a different opinion, which I respect, and that's what this discussion is about. It's also possible that I may have misinterpreted what Fediverse was supposed to be about, and I was just projecting what I would like it to be. I guess that we will see how will the Meta situation eventually turn out. I guess that Fediverse will probably split into Metaverse with instances that did federate, and Fediverse that didn't. All that remains to be seen is how large will be the communities left on either side. (Now that I think about it, maybe I have it wrong, and you can for example defederate from Meta, but still be federated with i.e metalovers.lm that are federated with Meta? How does that even work, actually? Will we be able to interact with Meta users through that instance, but not with their posts, or never message them?)