We're not comparing Marner at 21 versus Kucherov at 21, and who may provide more over the first X years of their career. We're comparing what Marner is likely to do over the 8 years of his deal, and what Kucherov is likely to do over the 8 years of his -- which cover the exact same time period so no need for cap hit percentages to compare different time periods.
Reality is, last year Marner had 69 points in 82 games. Kucherov had 100, with close to double the amount of goals.
This year, Marner is pacing around 110, Kucherov is pacing close to 130, and has 50 percent more goals.
Sure, you can make the case that Marner is closing the gap, but Kucherov should just be entering his prime, so he should still be improving as well. Until Marner figures out how to be a consistent goalscoring threat like Kucherov, he's unlikely to be better than him.
edit: With respect to the comparisons relative to other players that have had 40 assists in each of their first 3 seasons. Take Patrick Kane's 3rd season as an example... 2009-10. The median team was scoring 2.68 goals per game. This year, the median team is scoring 2.97. Scoring is up 10% over that period anyways.