You're not understanding that growth on fediverse instances run by Meta is ultimately bad for the fediverse in the long run.
Let me try explaining like this: Imagine there's an instance called meta.world and it gets hugely popular. Whenever you browse the all feed, it seems like 95% of the posts are from meta.world. Everybody hates that it ended up this way, and everybody tried to fight it, but it just inevitably happened because Meta has the fastest and most stable servers, and because there are a ton of funny users on Threads who only post to meta.world because Threads heavily favors those communities in their app. Then one day Meta decides that they don't want to support the fediverse anymore, so they close off access to meta.world. So effectively 95% of the "fediverse" as we knew it vanishes, and you have to join Threads if you want access to those communities again.
It's the threat of that scenario that has a lot of people wanting to block Meta from the start.
You say you understand my point about the threat of Meta consuming the entirety of the fediverse, but then you talk about privacy, which is completely irrelevant to my point. What does privacy matter when Meta gains 95% share of the fediverse communities and then closes them off to only Threads users? In that situation, your privacy is completely gone. You have to join Threads to get back in to the old fediverse that Meta took away.