Thanks for the alternate perspective! However, it doesn't really alleviate my concerns much:
Firstly, the claim "Nearly everyone here came from Reddit's API fiasco." isn't necessarily true, as the fediverse consists of more than Lemmy and the threads section on Kbin. Threads primarily concerns microblogging, which includes platforms like Mastodon, Misskey, Firefish, Friendica, and Kbin. Even so, you could say that all of these people came from similar situations — they all could've chose Twitter or what have you and instead chose the smaller platforms on the fediverse. But does that really mean that they'll stay here? This whole situation is showing that people are desperate for activity, even if that activity means relying on a corporation with interests that go against ours. People have already left following drops in activity — during Ernest's hiatus, the number of active Kbin users plummeted (yes, it was always going to down after the initial rise from the Reddit migration, but Kbin went down a lot, especially compared to Lemmy) and hasn't recovered. Those people, who have tried it and said, "Yeah, this isn't really working," are really hard to get back.
Now, imagine if everyone here got used to the large amount of microblog content provided by Threads, interacted with it a bunch, followed a bunch of Threads accounts, etc. like people are expressing plans to do. If Threads quits federating, thereby making all of that content and connection inaccessible to the rest of the fediverse, do you think people will stick around after that much of an activity drop? No, tons of people are going to follow the 99% of activity and flock to Threads, leaving the open fediverse in a position worse than right now. If you want to give Meta the chance to kill the fediverse's chance of growing to become a legitimate competitor, this is how you do it.
What you say about people having choice is true. If people want to access the fediverse through Threads, that is absolutely their choice. However, another beauty of ActivityPub is that we have the choice over whether we accept interaction with Threads or not. We don't have to become dependent on Meta to carry the fediverse — we can shut them out immediately and grow slowly but surely. If someone wants to join Threads, we aren't under any obligation to show them our content. Instead, saying, "Hey, if you want interaction with us, head over to something that cares about an open fediverse like a Mastodon or Kbin instance," is going to be much better for us long-term.
I don't think its right to view ActivityPub as competition to mainstream social media networks,
No matter how we view it, Meta views ActivityPub as competition to Threads and rightfully so. The values we currently hold on the fediverse — transparency from moderators & developers, no one instance having control, people having lots of choice, etc. — are values that directly go against Meta's profit whether they join ActivityPub or not. It is in Meta's best interest to pull as many users from here as possible and to nip the fediverse in the bud before it grows. Zuckerberg is not here to play nice, and I can't fathom how anyone would believe so. He does not care about a decentralized, open fediverse. He wants to get as many people on his platform as possible and to make potential competitors less viable, and this is how he can do it.