When a team is under-performing when going gets rough, maybe the leadership or team mental is not good enough. It was always strange that Matthews has C in his jersey, he is maybe the biggest choker in playoffs, and his aura doesn't scream leadership. For captain, you want the player who either works the hardest, makes clutch plays or who electrifies his team with something (a nice hard check for example).
This core and playoffs are pretty iconic duo, it keeps on amusing. But I'd actually trade Matthews and would let Marner go. It is super hard to get 1c and 1w, but still. You could get tons of pieces with trading Matthews, then it is a matter of luck to acquire legit 1C for the team.