They don't have to leave leave. They can just join here and reduce the time they give reddit. Little by little they can just get out of that and into other stuff.
I think the biggest impediment (besides the learning curve of the new format) is the fact that there are many more users and much more activity on Reddit. It will take time to build enough of a user base on lemmy to convince people to join.
That being said, the subreddit blackout starting June 12th will remove that advantage for Reddit. Even if most of the subreddits only participate for 48 hours, that might be sufficient to drive enough people to lemmy that it becomes perceived as a viable alternative.