The Leafs were 29th at 5v5 expected goals last year; that’s lottery team level right there.
They won the division because they had excellent goaltending and a much better defensive group.
Matthews doesn’t need Marner to exceed at this point in his career. I would say Marner needs Matthews far more than the other way around.
Marner plays slow, was incapable of getting in on the forecheck, so any winger capable of doing that is going to suit Berube’s systems.
With Marner gone, they can build out a balanced team, it’s really that simple.