No matter how good he is on defence you cant defend yourself to a win. You need at least some offence. Both defence and offence should be spread around and not on one player. That is to risky. And fragile. Then we got that chemestry part that i dont think andvaced stat can collect to its fullest. I am not against andvanced stats , i just dont think u should put all your eggs in that basket. Unerlying stats might be fantastic but the result poor. Wich suggest that over time the player will reach or get close to those underlying stats. But its is never a fact it will happen, And specially not exactly when you want it to happen the most (playoffs).
And no matter how unfair it may seem. you pay for offence among forwards. Not good enough from Marner when it is needed the most.
The problem is that it is realy hard to get the right players for that 12,5 13 mill he probably wants. Might still be better for the team in the long run, due to things not in advanced stats.
Him having his nerves on the outside might affect the team as a whole. But that is just specualtion from my own experiances with team psychology.
That said i belive he will stay.