I'm fairly new to the fediverse, but I see it this way. If Threads integrates with the Fediverse then users will become accustomed to the content from Threads, which could cause more friction in the future if there's a movement to defederate from Threads. I think it would just be easier to avoid all that in the first place. I also feel like if anyone really wants to see Threads content they could make an account there, and then everyone gets what they want. Then there's all the downsides others are theorizing could happen, but they can't be comfirmed until Threads actually joins the Fediverse.