This may be an issue of poor wording on my part. The idea I'm trying to get across is that if Kbin, Mastodon, & other microblog platforms that are currently on the fediverse don't defederate from Threads, the users on those platforms will become dependent on Meta for activity. If Meta then leaves the fediverse, activity will drop down to what we have now, only with people used to the activity brought by Threads and having all of the connections they made with Threads accounts. This will pull tons of people from Kbin, Mastodon, etc. and essentially kill the open fediverse with very little hope of it growing again.
And to be clear, Meta has tons of incentive to do this. Meta is a profit-driven company. They want as many people as possible on their platforms, not on Mastodon or on Kbin or on Firefish or on Misskey. The open, widely distributed fediverse that we strive for goes against Meta's motives. Just as they'd have everyone on the Metaverse if he could, Zuckerberg will absolutely pull people from Kbin onto Threads if he gets the chance, and he will get that chance if we decide to put the content pool in the hands of Meta for the sake of trying to get activity as quickly as possible.