Dach is nearly a year older.
Dach played almost 3 and a half minutes more per game (and significantly more time on the PP)
Dach only scored 22 even strength points to Lafreniere's 35
Dach's shooting % this season was nearly double that of his previous three seasons (which had been pretty consistent, making this year look like a aberration)
Laf's shooting % this season was a third lower than the previous two seasons (again, making it seem like this year was the aberration)
Dach plays with Montreal's best players. Lafreniere plays with the Rangers' 3rd line.
Ultimately it doesn't matter. Laf is not a bust. If he never improves from where he is right now (which is a stupid thing to assume of a 21 year old player), he's still an NHL player. No team is going to offer more than a low-ball or, at best, a lateral trade for him, and the Rangers have no need to accept even fair value, as they would already have a player of that caliber (Laf) who has a chance to become even more.
Running out the current group (with the vets taking up all the minutes and starving the kids of the ice time they need to develop) doesn't work. I'd rather retain on Panarin, bring back a bunch of assets by moving him, and thereby free up the minutes Laf needs on the PP and in the top six. If Laf still doesn't develop, oh well. It isn't like the current group was going to win the Cup anyway, and at least then you'd have the assets in the fold from trading Panarin.