A huge number of people who read reddit posts aren’t actually reddit users. Reddit has become just as important a repository of internet knowledge as Stack Overflow or Wikipedia. If you’re having trouble with software or something technical, the information on Reddit is invaluable.
That information does not exist on Lemmy Lemmy as an alternative when it comes to that information that already exists in the world is not a viable alternative.
The best possible thing that people moving to Lemmy can do is leave their comment and post history up, but continue to provide good answers to questions here on Lemmy. Eventually the knowledge on Reddit will be here or outdated enough to fade into obscurity.