This is just fear mongering to me. Just because some people defederate with some other people, does not mean everyone will suddenly start defederating with everyone.

That's just a slippery slope argument, and no better than the rest of them. There is no reason the behavior would suddenly spread everywhere, especially given how much of the Fediverse shares your exact opinion.

I haven't seem any polling, but it seems fairly close to a 50/50 split between people who strongly disagree with defederation vs those that do not. That is plenty of people to keep most of the service federated.