If you’re already up and running on Mastodon and can interact with people on Threads, there’s literally no reason to swap one for the other.
This is about encouraging new users to join Facebook instead of one of those other Mastodon instances. Realistically, what percentage of people who join Threads will consider joining Mastodon or an independence instance instead when Facebook decides to drop support for Mastodon federation? I would guess that number at 1% or less. In other words, 99% of all Threads users are stuck there for the entire term of their service, never actually joining Mastodon.
The point of Facebook investing all of this money into setting up Threads is to eliminate competition from decentralized services. They are terrified that they are losing all of the control over the Internet that they have slowly acquired over the past 15 years or so, they are trying to take it back and destroy the competing network of federated independent services.