say things keeps growing nicely and then the next reward split rolls around and half the miners decide to shut down their operations overnight because it's not profitable. It might go from 10 minutes to find a new block to 2 hours or more. Then it could take 168 days to reach 2016 blocks and have the difficulty readjust.
The same, or similar thing was a concern the last reward split. We've seen what has happened since then. I would expect similar in the future.
The sky hasn't fallen yet.
Sam