I don't know why you'd do that trade. As others have stated, if you're trading the Moose, you're trying to free up cap space. Trading for Marner doesn't do that, unless you're going to let Marner walk in 2025.
Also, Rants' effort level has been bad for two seasons? His effort level has been uneven for way longer than that. It's who he is. Ya just gotta learn to accept it. That said, I don't know why you'd cite LAST season as a problem. When the team was losing bodies left and right and there was no center depth to speak of, Rants stepped up. Slag him all you want for this season, but it's kinda silly to diss him for last year's effort. He was arguably their best player, he most certainly was for a good stretch there when they almost no other impact players in the lineup.
Lastly, I don't care how talented he is, I want absolutely nothing to do with Mitchell Marner. For a guy who got paid superstar money to play for his hometown team, he sure does come off whiny and petulant. At the risk of sounding cliche, the best players in any sport meet the moment, and he never has, despite oodles of talent and hockey sense to spare. Mikko, on the other hand, has.
So much talk about how players live for the playoffs. Remember, they're literally playing for free right now. Marner comes off as a guy who DREADS the postseason. There were like, two consecutive years where he was so goddamned nervous he launched the puck out of play. And I'm not even talking about a Patrik Nemeth long bomb that goes just over the glass on the far end. He howitzer'd it into the frickin' rafters like he thought the net was near the scoreboard. For such a smart player he sure does play dumb after Game 82.
Would a change of scenery do him some good? Probably. But that's not a guy I'd ever bet on. He's not the primary reason the Leafs continually faceplant in the postseason, but he sure as f*** ain't the solution either.