This is exactly how Zuckerberg wants you to think.
These conversations we're having are all speculative, and we won't know how things play out till we get there. Trying to predict the behaviour of large groups of people is... difficult
What I predict is that defederation will play right into their selling point. We're going up against a behemoth of evil with enough money to bankroll creators into joining and promoting their platform. Defederating (when the majority of people don't understand what that means) will end up with people joining Threads.
Threads has a very high (artificially inflated) user count, it's by a company everyone already knows, and all instagram users already have an account. The strongest selling point we can have is "Join Mastodon, you can see all the same stuff but it's run by a non-profit instead of Facebook" That doesn't work if the selling point is "Join Mastodon to see different content".
For what it's worth, I'm actively using Mastodon and trying to inform any friends / family that are jumping ship to shift to Mastodon. Best case scenario, Mastodon takes off properly, Threads becomes a failed project by Meta, and we can nail this shut for good.