Present value outweighs future value. This is why picks this year are heavily weighted over picks next year. Also, the Flyers assumed all of the risk in the deal. Furthermore, not only did the Flyers lose the current value of Schenn, they're burdened by the fact that their coach actually negatively impacts his team from time to time by putting Lehtera into the lineup. Regardless of his flaws, Schenn was a productive player.
The fact that Couturier stepped up his production this year doesn't offset the loss of Schenn. If Schenn were here, more goals and wins would have resulted.
I'm pretty excited about Frost, but Schenn was an exciting prospect at one time, too ("best player not in the NHL"). The Flyers suffered through his growing pains and traded him as he entered his prime. If you make this trade every year with your 25-year-old 25-goal scorers, you'll never win a Cup.
I'd label the deal right now as "fair to OK," with a chance to be good. But that's a long ways off. If we have to wait 5 years for Frost to be as good as Schenn is (which is a really strong possibility), I'd call the trade a loss. The average value of a middle 1st-round pick doesn't offset 5 years of value.
If Frost is only as good as Laughton, it's a disaster.