As for the Leafs, they have a ton of offensive firepower already, so Karlsson would be a decent replacement for Marner, however where they would truly win here is on the back end. Toronto has failed to make deep runs in the playoffs and have some weaknesses on the back end. Pietrangelo would immediately bring stability, a veteran presence, and tons of playoff experience.
People are forgetting that Pietrangelo not only won the Cup with Vegas, but he did so as well with the St. Louis Blues in 2019. That's exactly what the Leafs need if they are going to get over that playoff hump. As for Vegas, Marner will be a huge upgrade over Marchessault. The verdict is in, this trade is a win for both teams!