I doubt he ever becomes that good, but then again I doubt Zadina scores 40. The odds are similar on Kotkaniemi being a Kopitar like player and Zadina scoring forty. And I know which one I want if it pans out.
I genuinely don't see why anyone would doubt Zadina scores 40.
If you meant this year, sure, okay, I agree. But I'm interpreting your comment to mean you believe he never hits 40 goals at any point in his career, and that seems bizarre to me.
8 players scored 40 goals last season:
Ovechkin, Laine, Karlsson, Malkin, Staal, McDavid, Seguin, Lee.
Zadina got 44 goals in the QMJHL last season. He was 17 at the start of the season.
Ovechkin got 8 goals in 40 games in Russia at the same age. Ovechkin didn't touch the NHL till age 20, at which point he got 52 goals in his rookie season. Hard to know what he'd have gotten in the QMJHL at age 17, but I doubt it'd have been higher than 44 goals if he only managed 8 in RSL.
Laine got 17 goals in Liiga as a 17 year old. 36 as a rookie in the NHL. He was 18 as a rookie though, so this upcoming season for him is actually the same age as Ovechkin's rookie year.
Karlsson at age 17 got 20 goals in 38 games in J20. He didn't even make it to the AHL till age 21, and the NHL till age 22. He also didn't get over 10 goals per season his first three years in the NHL.
Malkin got 3 goals in 34 games at age 17 in the RSL. He didn't enter the NHL till age 20, at which point he got 33 goals in his rookie season.
Staal got 39 goals in the OHL at age 17. He then got 11 goals in the NHL as a rookie, before being sent down to the AHL for his sophomore year. He was 20 when he finally realized his goal-scoring potential and had 45 goals in his third year or professional play - a number has never beat since.
McDavid had 44 goals in the OHL at age 17.
Seguin got 48 goals in the OHL at age 17.
Lee was still in the AHL at age 24.
The only players played in the same tier of league as Zadina at the same age were McDavid, Seguin, and Staal, Zadina outperformed Staal, tied McDavid, and was slightly behind Seguin.
Many of the others were clearly much worse than Zadina at a similar age. Zadina will almost definitely start this year in the AHL, but he'd have to play the next 6 years in the AHL to be on the same developmental pace as Lee, the next 4 to pace Karlsson.
I personally expect Zadina starts this season in the AHL, but gets called up at some point and finishes the season in the NHL and never touches the AHL again once called up, some people might consider that optimistic, but I think very few people will disagree with me that he will be a NHL regular at a younger age than most of the people mentioned above were.
McDavid is the only person among last years 40 goal scorers to have played in the NHL at age 18. I see no evidence that Zadina is a dud, or should be expected to never hit 40 goals. He's young, players his age do not typically play in the NHL, not even future 40-goal scorers.