Of course. Lafreniere or Slafkovsky could regress. But without concrete signs of regression, you may as well say the equally vague "Anything can happen".
Eh, not really. You're making it out to be like some lottery ticket. Neither will have to regress, they're both probably going to progress. However, it's perfectly plausible without saying "anything can happen" that Lafreniere will progress more. I've said this a number of times he has a very valid claim to be the second best forward on a cup contender. And I know they're both a few years in, but would it require that much imagination for the guy that was the much better prospect at the time of the draft to end up better?