Actually the Penguins winning in 2017 with Letang not even available to play kinda blows a giant crater into the whole "you can't have X players earning Y cap!", because that means they effectively won with their best players earning about 59% of the cap due to Letang being a 7.5 mill cap hit that didn't even play a game in the playoffs.
The Leafs problem isn't their salary structure. It's that Matthews and Marner and Tavares don't show up to play big in the playoffs. The Oilers big three of McDavid, Draisaitl, and Bouchard have some of the highest PPG in the playoffs in the modern era (last 30 years lets say). McDavid is no.1, Draisaitl no.2, MacKinnon no.3, and even Bouchard is just a hair below Makar and near Sidney Crosby himself, way above any of the Leafs big three forwards.