Without delving into why the idea of Mack > McDavid due to contracts is idiotic - one dynamic i'll bring up that rarely gets discussed when comparing the 2 players is this:
1. Put McDavid on any team in the league - Montreal, Arizona, Tampa, Toronto....i'm confident he produces as much as he does now, maybe a bit less/bit more.
2. Put MacKinnon on any team in the league - like Montreal, Arizona or Detroit for example - and i'm not sure how he does. It took him a few years to get going, and his chemistry with Ranta obviously has helped propel him to a new level. I'd have much less faith in Mack's ability to replicate that level of play outside of Colorado than I would McDavid in a similar situation.
Bottom line - as fantastic as MacKinnon has been as a player the past 2 seasons - he's probably not worth overpaying for in a trade as I feel there might be some risk in him replicating his play elsewhere. I don't think there's a similar risk involved if you overpay to trade for McDavid