It might take time but why not naturally wait for these famous people to expand their network and consider being on lemmy/mastodon? Without having to interact with Threads at all? I feel it would just be a matter of time, and we can adjust on our own terms instead of having to let Threads in.
On the contrary: Advising popular instances to defederate with threads will actually help promote diversity of instances, since the Threads population is orders of magnitude a larger network than Mastodon. Having popular instances defederate with threads will keep lemmy from actually just merging into the threads network.