I’m not sure I understand what you mean? That people would migrate away from Threads if they knew they only knew can access their followers from somewhere else? Why on earth would that ever happen?
I mean, that’s what happened when people left myspace for facebook. Facebook provided tools that let them use facebook while still communicating with myspace users until enough people switched and myspace became irrelevant. Myspace’s dominance was unassailable until it wasn’t.