It's reductive to say every negotiation has a winner and a loser. It's possible for all sides to come to a mutually beneficial arrangement, and I think that's the case or something close to it with most NHL contracts that get signed.
Kyle Dubas lost the Mitch Marner negotiation.
Mitch Marner is a very fine player. I profiled him in our Top 25 Under 25. The Leafs are a better team with him on it, and he is young enough to keep improving. Between the departure of Mats Sundin and the arrival of Auston Matthews this team probably had zero players as good as Marner.