That's not how people work. If they start from Threads, very few will switch to a 3rd party client. And defederation will happen anyway once Meta gets control, it's the whole point of EEE.
You do have a point though- Threads could be a threat to Mastodon even completely isolated. A lot of current Mastodon growth isn't because of its draw as a product/platform; it's simply people people leaving Twitter for something else. Threads will also be a something else, creating meaningful competition