It's simply hard to project prospects and a lot of times it's about timing and opportunity. There's also the fact that many posters here (including me most of the time) value outside of the NHL "potential" more than NHL contribution. It's like NHL experience without much progress is somehow more of a death knell than a prospect who has been unable to crack the NHL. We see this with the Shinkaruk for Granlund trade. Even to this day some posters here refuse to call it a good trade for reason that it was "unnecessary." That "possibility" of a prospect reaching their drafted potential in the NHL is perceived by many to be worth more than a player who is an NHL player now but who hasn't shown the potential that they displayed as a prospect.
Take Dale Weise. When he was here we saw a guy who had size, could skate, and had good hands. But he was stuck on the 4th line playing limited minutes and producing solid 4th line numbers (which in the grand scheme of things is very average numbers). Since he was 25 and didn't improve his numbers over his 2+ seasons here many here thought he was a dime a dozen 4th line player. He goes to another team, gets more opportunity to play, and he proves himself as a good 3rd line player. The same fans here who are raving about Kassian now were critical of the Canucks for trying to make Kassian fight and play a physical game rather than giving him a chance to be a top 6 producer. But the game Kassian played this past season is what the Canucks tried to get out of Kassian. Kassian had 7 fights in the regular season this season but apparently Canucks management were incompetent in forcing Kassian to fight and play like a grinder.