Regardless, you're using / not understanding the mathematics incorrectly.
The plot says that the average player peaks at age 23 -- and for what it's worth I am willing to completely believe that statistic. Let's assume that the average player peaks at age 23. Note that this includes a lot of players like Galchenyuk and Poehling, who peak before 23.
But we're not discussing the average player, we're discussing Nylander. Nylander is 27 now ... And he's peaking at 27 and counting. He hasn't declined in the past four years, he's actually gotten substantially better. So he's already an outlier.
Second, a long term contract already accounts for depreciation. If Nylander is signed for 7 years, and the cap increase is by 5%/year, then the contract at fixed cap hit allows for a 41% decrease in value.
Third, Hughes isn't supposed to just accumulate players, he's supposed to build a team. Every single championship team has players older than 27, so take a hint. A good GM needs to be able to identify the outliers.