The valuation is applied against the cap at time of signing because that is often the only cap known, and the "current cap" has everything to do with the "current signing".
We're in a unique situation right now, where there is known information about the future cap, but that is unusual. Nylander could choose to wait and sign against a higher cap, given that he has this information that most players do not, but there are risks to that choice, and many players choose to lock in their guaranteed generational money and stability for themselves and the team. What many people like to do is take a contract that was signed without any known future cap information, and then apply that signing to a cap that never existed or was expected to exist when it was signed, which is obviously wrong.