I might not be understanding something about the fediverse, but isn’t this like if you couldn’t send or receive email from a friend because you use gmail Google decided to block @icloud domains?
Like, you’re not forced to follow or interact with it, and are free to block it if you can think of a reason to, I just don’t see why such an open platform used to limit users in this way
The concept as I understand it is that Threads has the sheer volume of content to completely drown out the existing Fedi content if it fully opens the floodgates. If that occurs and say 90% of content becomes Threads and then they start making Threads only extensions to Activity Pub, servers will have to start patching those in and the Activity Pub project is defacto owned by Meta.
People also have issues with the Meta content moderation and the population on Threads, but as you noted that’s fixable on an individual and community level. The existential threat to the future of the Fediverse is why servers should defederate. Meta can’t and shouldn’t be trusted with any amount of power over this community project.
This is called “Embrace, Extend, Extinguish”. Microsoft coined the term internally for their responses to open standards in the 90’s and 00’s.
https://en.m.wikipedia.org/wiki/Embrace,_extend,_and_extinguish