The concept is that overtime, communities and connections will organically grow. If Threads has a disproportionally large ubserbase, then overtime they will create a similarly larger number of communities. This then would give them a lot more influence over the fediverse and anyone federated with them.

For smaller instances that become accustomed to seeing those communities and content, the danger is that Meta can just “pull the plug,” defederate, and extinguish the competition, or at the very least hurt their competitor's users experiences when interacting with content from Threads. The reason they might do this is purely because it fits into their business model which is selling user data to advertisers; it is in Meta’s interest to have as much data on their users as possible, and to have those users be based on Meta’s platform.

As I said in another comment, I could be totally wrong and this could benefit the fediverse. I just think the opposite is more likely because I do not trust Meta. I think they will play nice in the beginning, but then start to flex their muscles once they feel they’ve got enough influence.

Also, there is nothing stopping them from expanding Thread’s capabilities to include the threadiverse. Kbin has already demonstrated both are possible in one app.