Easy- unless you're Mcdavid, Drai, Mackinnon, Crosby, etc- most NHL forwards follow a similar trajectory. Their most productive years are between 24-29, then a slow decline from 30-34, then a faster decline 35+. The guys that have been underwhelming- Skinner, Arvidsson, Henrique, are all 32-34, all in line with average player decline. Even Kane's decline the last year could be partially attributed to age, same thing we saw when Lucic was here. The guys who left- Mcleod, Foegele, Holloway, are all 23-28- a forward's prime. When we traded away Hall and Eberle they were in their mid 20s too, no surprise they continued to produce until (surprise!) they hit their early 30s.
We develop players for other teams, give them away as they hit their prime, then sign players who's offensive prime has passed them and wonder why they don't produce like they did. Nobody defeats time, nobody.