You seem to be missing the fact that the information available also included potential growth. This was always a big possibility. He went "safe" and it'll cost this team significantly.
The biggest problem I have with this argument is that it ignores the fundamental and irrational value shift that Marner's camp pulled.
Projecting forward from last summer you can say-
he might take a step forward, and after a comparable season to
Gaudreau, he'll have earned a cap adjusted 7.7x6 that scales to 8.7 x 8 as a player that has proven ability to flirt with point per game and still potential for more.
or even
he might take a step forward, and after a comparable season to
Kane, he'll have earned a cap adjusted 9.18x5 that scales to 10.68 x 8 as a player that has proven MVP level ability
The key to both of those scenarios is that those are the proven values, locking in before hitting upside never yields full value for the upside because you're splitting the risk with the team
The 9x8 ask would mean valuing a guy coming off a 69 point season higher than a proven point per game guy with a with MVP potential, and it's really only an issue because he had the borderline MVP season and is now apparently rejecting that valuation.