And you don't need to look too far to see it now. The majority of people posting on the threadiverse come from lemmy. world and kbin.social. Why? Because they were already the established "big instances" and they went there.
The average user will just go to wherever is the biggest and that will be threads. And when they have the majority of new take up and most people on the other side have the majority of their contacts inside the threads world, then they don't need us any more.
This is just the way a corporation operates. Source: I work at a multinational corporation.