A big question is how much does Marner care about his legacy? Unless the team breaks through this spring (a megahugemassive if obviously), if he leaves now after 9 years, he'll always be known in his home town, the capital of the hockey world, as the guy who was assistant captain of a team that will be remembered as the biggest playoff failures in team history, and one of the biggest (if not the biggest) playoff failures in NHL history.
Or he could stay, and if he helps turn things around and go on even one deep playoff run, most fans will forget all the playoff losses, and he will compile enough points to own multiple all-time Leaf records and of course take his place on legends row.
If he believes there's even a small chance that this team can go deep, then staying is the obvious choice, because the upside is just so humungous, he still has a chance to make all his childhood dreams come true. Take a ton of money, plus millions in endorsements that he won't get anywhere else, and have a chance to be a hero in capital of the hockey world forever, also not possible anywhere else. And if he does have success, those endorsement dollars will keep rolling in as long as he lives, most likely that means tens millions more in his pocket so leaving would be an incredibly stupid move financially speaking. So why would he leave? Who would leave and leave that possible upside on the table?
The worlds biggest wuss, that's who. If he leaves, that sends the message that he doesn't think he has any chance of winning here so he just gives up, says "no mas", and goes down as the biggest wuss in team history, maybe even the biggest wuss in NHL history.