For one they are top heavy, and we've known for a long time that will not bring you long term success in the playoffs. After that the guys at the top they are paying never show up in the playoffs, so its a double whammy that insures that they will never get out of the first round if they match up with a balanced team.
Best thing they can do is not pay Marner, let Tavares time run out, then use those dollars to balance the roster out.
They slit their throat signing Tavares because they didn't need him and by giving him that kind of money it insured the younger guys were never going to play ball about taking less money. So not only did Tavares eat up 11 million in cap they could have turned into two guys to balance the rest of the team out, they ended up losing out on probably a good solid 2 million per contract on Marner, Mathews, and Nylander.
Finally some of their depth is exactly the same as those guys, aka Jarnkrok.