Pastrnak got paid for being a guy that had 60 points on non-empty netters in one year after not showing much in his previous seasons. That's what they paid for, and that cost a bit less than what would be a $7M contract today. He then exploded development-wise.
Nylander's 10 closest comparables from the past decade would even out to a slightly higher than $7M cap hit today. Their average ppg is a bit lower than what Nylander has, and all of them played bigger minutes or with as good linemates as Matthews and Hyman. Since their contracts, they average about 70 points per year. If Nylander hits that, he is perfectly in line with what that kind of money buys from a post-ELC stud.
As for your earlier points, expecting 80 points just because he plays with Matthews is absurd. You don't hit that kind of numbers unless you get big minutes on a top PP unit. He plays with Matthews at even strength, it only helps him there. And with Matthews, Nylander was one of the best producing RWs in the league at even strength. But he also played on a crap second PP unit, which limits what kind of production you can get out of him.