Pretty clearly a lot of the big Fedi instances are going to go ahead and federate Threads. I see 2 possible outcomes.
1. Threads accounts which abuse Fedi citizens get quickly suspended/blocked, i.e Threads/Fediverse moderation works.
2. The moderation doesn’t work, and Threads is quickly defederated from most of the Fediverse, just like Gab.

Neither outcome is terrible? #2 shows the world that federated social media can defend itself, a useful lesson.