Reddit is too big to fail, they have achieved critical mass. Keep in mind facebook is still around despite being a reviled company, and instagram certainly hasn't had a mass migration off of the platform either.
At the end of the day Lemmy isn't a replacement to reddit yet. It depends entirely upon it getting traction which thus far still hasn't occurred - we are not at critical mass yet. I hope it happens but there are many reasons why this site could fail even after reddit's admin blunders. Too many people are apathetic to the changes and not all of them are lurkers who do not post or comment.
Today you can't just stop using reddit either, especially for google searches. Too much content is ONLY on reddit. It's a huge problem. We really need a wikipedia style reddit where it's not for profit and still moderated for content.