That's some fine digging, that said if they had gotten Mitch signed to that we'd all be in hog heaven right now in possession of the NHLs best contract. From some of the scuttlebutt didn't Dubas try to get him for 9, right before Dubas gave 34 the walk away deal.
Dubas actually did try to negotiate with the Marner camp before Tavares.
Marner scored 69 points in 82 games that year and led the team in scoring but the majority of his points came in the second half of the year where he was scoring at a 90 point pace - a pace that he's been scoring at ever since. The first half was largely spent in Babcock's doghouse.
After the year was done, the Marner camp reportedly asked for somewhere in the neighbourhood of 8-9 million for 7-8 years for an extension. They were betting on the fact that Marner was more the player he showed in the second half of that year than the first half, and wanted to be paid a premium based on his potential. Otherwise, that'd be a pretty significant overpay. The Leafs said no and the next season he put up 94 points and was able to command much more. This story would have been much bigger news but the Tavares signing completely overshadowed everything else that summer.
If you also recall, that was the year that Point and Rantanen also were negotiating for new deals and like Marner both guys chose to wait until the following year to extend.
Since that contract, you'll notice that most NHL teams now will elect to overpay a player a year early and bet on potential rather than wait and have to pay more for results later or go through a protracted contract dispute.
Examples:
Clayton Keller - Got a $7x8, coming off a 47 point season. He is now putting up a point per game, deal is a bargain now.
Nick Suzuki - Got an $7.8x8 coming off a 41 point in 56 game campaign, though he did have a good playoff. He still hasn't earned his AAV yet but he's show significant improvement since.
Jack Hughes - Got $8x8 coming off a year where he put up 31 points in 56 games, worse than Suzuki but his underlying numbers were far better. This deal paid off almost immediately. He might be on one of the best contracts in the league now.
One that might be a backfire could be Barzal - extended at 9.1x8 years and he hasn't been a point per game player since his rookie season but that deal doesn't kick in until next year so who knows.
Lesson to be learned here? Extend your young players a year early rather than having to pay them a year later when they are likely going to be much better.