Open source, non-corporate instances should be able to grow, and that growth will be stunted if most people who want to interact with the fediverse are deciding to go to corporate, profit-driven instances.
The issue is, how does defederating not promote leaving for Threads or instances that federate with Threads?
I think it's a good argument against Threads federating at all, but a poor one for defederating from Threads.
If Threads produces 95% of content in the fediverse, and your instance defederates from them, then your instance just doesn't have access to those 95% of content. Threads and its friends will be a lot more attractive then because it has 19x the content of what you have access to on your instance.
I think this will still lead to people leaving for the threads fediverse.
Also, I get the argument for Mastodon, but does /kbin actually have anything at all to fear here? Sure, the user numbers and content would be way higher than the rest of the fediverse. But Threads is a Twitter contender, not Reddit like /kbin and Lemmy. We will only see their content in the microblog tab.
Is the microblog tab actually that important to most people, that the instance could become dependent on Threads for dominating it? I honestly don't see it happen, I feel like this is an imported issue from microblogging platforms that's just repeated here despite being a non-issue for us.