Here's my take. First - Marner and AusMatt are 2 of the best players in the league and worth every penny of their paychecks. They just got checked into the ice by Habs and were muzzled through much of the series.
Leafs were a 2 line team in much of the season. With their #1 line checked, they got a little scoring from the rest of the team, but not enough. The Tavares injury hurt badly, because that caused the rest of the forward lines to be scrambled. Willy produced for much of the series. But in the end, Leafs could not put the puck past Price enough and they were vulnerable on the transition.
They need more depth. Same thing the Oil discovered against Jets. You can lean on Dreiseitl and McD throughout the regular season, but you need secondary scoring in the p'offs.