The biggest mistake on the Leafs isn't the dollar values on their contracts. That's a contender for biggest mistake, don't get me wrong, but the real culprit for why we always lose in the first round is that the inmates run the asylum. Dubas hired Keefe, who never made the players accountable. Dubas was part of the decision to make Tavares Captain. Marner's ego is what functionally runs the team, and it's the weak leadership structures above and around the players that cause our core to crumble every year.
I'd love for us to have money for a goalie, or another top4 D, or whatever you could buy with better contracts for the big 4. But far, far shittier teams than us have won rounds, or even multiple rounds. If our problems were just that we had an extra PPG C for a few years instead of a better goalie and a 2RD, we'd have at least run some rounds. No, our problems are that our core are a bunch of basket cases when the other team play like meanies.
The mistake wasn't giving Tavares money and as a result giving extra money to Matthews and Marner, the mistake was giving Marner the keys to the kingdom and a coach who had to apologize on TV every time he said the core was kind of ass.