You say: The people who are at currently at this point have already gone to Threads. Then you say that if traffic from Threads is subsequently withdrawn, all the people who haven't already gone to Threads will... go to Threads.
Let me clarify. When I say, "The people who are currently at this point…," I mean the people who right now feel that they need to interact Threads. If they do, they're probably there. My issue is that if people are dependent on Threads for the vast majority of microblog activity, more people will feel that they need to keep that interaction with Threads. I'm not seeing how this is some far fetched theory more than it is straight up inevitable. If activity increases by 50x because 98% of the content is now coming from Threads and most of whom people are following are on Threads, more people will feel the need to stay connected. I don't see how it could be otherwise. This means that if an instance wanted to defederate from Threads for any reason or if Threads defederated themselves (which they have tons of incentive to do later down the line), tons of people would leave.
To give you an example, imagine if kbin.social was to defederating from lemmy.world and lemmy.ml due to unhappiness with their moderation. Obviously, defederating from any instance is going to lose you some users, but those two instances harbor a massive portion — probably a large majority — of the content on the threadiverse. Tons of people would leave kbin.social for the simple reason that most all of the activity that they were used to would be gone otherwise.
Now, with Threads, there is some resistance in the fact that Meta is a massive for-profit corporation. Many people won't move to Threads on principle. However, this is countered by the extremely strong pull factor of the sheer percentage of activity Threads would harbor. If people get used to all of that activity based on Threads and are following mostly Threads accounts, tons of those people will leave an instance should that instance defederate later on or jump ship from the fediverse to Threads should Meta cease federation. And among those who don't leave, there will likely be a lot less motivation to post after such a drop in activity and interaction.
I don't see how dependency on Meta for the vast majority microblog content could possibly be a good idea. If Kbin were to implement a silencing feature like what Mastodon apparently has, where Threads content would be invisible outside of Threads users that you've followed, I think that'd be fine. That way, people could intearct with a few Threads accounts they're especially interested in as opposed to the public microblog feeds being 99% Threads and us being dependent on Threads to maintain the activity of those feeds. But just letting them flood our microblogs seems like an extremely dangerous idea that's wholly unnecessary, and I haven't been convinced otherwise.