Google was not charging people to talk on their network, and they didn't make it harder to reach someone once they got it. So there was no reason for people to jump out. Facebook, on the other hand...

When the internet was in its infancy, companies and small businesses first established their online presence by getting a aol.com or hotmail.com. Running your own email or website was still expensive and not something easy to do. Today, having "your own" social media and being in control of your brand is almost as easy as having your website and your domain. I am not saying that everyone will jump out of Threads, but if Threads ever gets successful enough to replace Twitter and if we don't shut them out of the Fediverse before it happens, at least there will be an opportunity for small businesses/media orgs/influencers that want to keep reaching their audiences (like they do today on Twitter/Facebook/Youtube/etc) and also want to take control of their own presence.