I don't see this necessarily as what's happening.
Let's say for arguments sake that Marner's best comparable league-wide is Kucherov at 9.5/8
If the Leafs were offering him say 10/6 with all the perks of up front signing bonus money and all that jazz, that would be by definition a very player friendly deal and frankly an overpayment based on his comparables. The idea that it has to be Matthews' money however, if that's what is actually the ask from the Marner camp, doesn't represent simply a player friendly deal, it represents a probably 30% overpayment. This is where things break down imo.
Matthews 30% overpayment based on just 5 years = Marner overpayment by 30% on similar term.. Fair is fair from a players perspective.
Two wrongs don't make a right, and neither Matthews nor Marner should be 30% overpaid to comparables as that is bad for competitiveness and that hurts Cup chances.
However if JT makes $11 mil and produces at 1.07 PPG rate, and Matthews gets $11.634 at 1.07 PPG rate, then your teams leading scorer who produces offense at a 1.15 PPG rate has a valid case to requests similar pay for similar play, while all playing on the same team together.
I don't see that as player greed, but rather management creating this internal salary structure freely of its own doing, and now suddenly realizing cost constraint has to happen and making it the players fault to do so or the team will not be able to be successful going forward. If I just gave two of my own the keys to a new Ferrari, then what would you expect was going to happen with the 3rd?
Even if Matthews is overpaid today at $11.634 mil, had that contract been for 8 years then Marner can't be asking $10.5 mil X 5 even when using Matthews as a direct comparable. Past precedence set term and price and now it appears management in moving the goal posts, recognizing they can't keep making the same mistakes over and over.