Marginal value is a fundamental economic concept that has infinite applications both inside and outside of hockey, not a single hockey stat that can only be used in a single way. You can absolutely apply the concept to how important a particular position is within a roster. Goaltending is the most important, followed by defense, followed by forwards. You can break that down further to RD/LD or center versus wingers, or how valuable a 3rd liner is versus a 4th liner.
You also have to look at the marginal value of players not just adjusted for position, but adjusted for contracts. McDavid's contract is substantially more expensive than the vast majority of other players in the league. So you're paying 50% more for a guy who might only be 10% more valuable to a team on his own- so do you want to pay that, or do you want to split that extra money across 5 positions lower in your lineup where the additional dollar buys a much larger increase in talent? The difference between Aho and McDavid is currently roughly $4 million. McDavid is better than Aho, but is that gap in talent more than the gap in talent between a guy paid $1m and a guy paid $5m? So you do you spend that much to upgrade one forward, or do you spend that additional money to pay your 3rd D pair, who spend 15 minutes or so on the ice each, an additional $2m each? What buys you the most value to a winning roster?
Those are all questions of marginal value. If you only think of it as comparing one forward to another forward, you're entirely missing the point.
