And I think that will go both ways. I mean, we all already have the option of joining threads right now to interact with those 100M users but I have a feeling most that are here aren't.

Their joining the fediverse will be more disruptive than their leaving it I think. And that's not even considering the higher costs to anyone running instances, since all that extra volume won't be processed and stored for free (though admittedly I am not familiar with the implementation details of how federated content is handled).