If you're expecting everyone to leave Reddit, you're going to be disappointed. Most Reddit users do. Not. Care. They'll stay for as long as Reddit entertains them.
The Twitter migration was actually a really great thing for the Fediverse. It diversified Mastodon, and made it an actually lively space. It's still a nerdy space, but it's so much more than it was. It's a genuinely general and engaging microblogging space. And while, yes, it doesn't have everything that draws the Twitter clout chasers, celebrity watchers, and journalists or politicians, it's a viable alternative for people who are looking to actually engage with each other.
The same is true here, and will be true after tomorrow.