Comparing Kassian's production to others' is fine, but drawing conclusions based on that =
Every player develops at their own pace. Just ask Burrows - 9 points in 81 games as a 25 year old.
Every player is in a different position relative to the team they are on. Seguin would have been a 1st-liner as a rookie if Edmonton selected him, and Hall would've been learning NHL defense in the bottom 6 in Boston. Van's top 9 forwards may not all be great, but they are at least decent and have a fair amount of experience - not "easy" to crack the top 9 by any stretch.
Kassian has the hands, the passing ability, and toughness. He just reads the play a little slow at times and needs more confidence. Maybe he never puts it all together, but none of the players he is being compared to has any influence at all on his future or potential.
Anyone that didn't see Kassian's improvements obviously wasn't watching very closely. Yes he was up and down, invisible for stretches, but he did string together a handful of very impressive games to start the season - something he never did last season. Tough to say how much his bad back factored in too, back problems can be very restricting. He was invisible last playoffs, but was more noticeable this time despite no points - only 6 Canuck forwards recorded a point FWIW. Last year he had a few shifts with the twins, this year he was getting more. He was even starting to get some shifts in the playoffs with them, and not just after Burrows was fresh-off the PK.
Consistency and confidence - chances are he finds more of both as he matures, leading to more icetime, then watch his production rise. Nobody knows how far he will go, but I can guarantee that comparisons to other players mean absolutely squat as far as Kassian's future production. When he was 22, nobody predicted that Bertuzzi would reach the heights he did - NOBODY... and who knows what would've been if not for the Moore incident.
If he puts in the work, and just keeps at it, the sky's the limit... not Crosby level or anything, but being a legit top-6 forward is far from out of the question. Labeling him a 4th liner at best... that is just stupid at this point.