pretty sure that is exactly his logic, yes
The logic is that you have the core 4 (plus Reilly, I guess) You've had what, 7 kicks at the can with this group? And in all that time your only "success" was barely beating Tampa and then getting annihilated by Florida. Otherwise you were consistently losing and even lost to a couple of teams you never should have lost to. So clearly, the core isn't getting you there and instead of running it back for an 8th try, a sane person would say that it's time to try something else.
So who, among that core, do you decide to flip for other assets?
Matthews? Not a chance.
Nylander? Why? He isnt overpaid, he consistently produces and he actually shows up in the playoffs.
Tavares? No one is going to give you anything good for him. He's on the wrong side of 30 and has been a loser his whole career. The Islanders never made the playoffs with him, but went to the ECF the year after he walked.
Reilly? Like Nylander, he isnt overpaid and he's your best defenseman on a team that doesn't have a particularly good blue line.
That leaves Mitchell, the petulant man-child. The player who never elevates, never shows up in games that matter. An elite regular season player on a team that can easily win regular season games without him. He is the obvious choice to trade away for several reasons.
First and foremost, he is overpaid. To spend that much cap space on a player you don't really need in the regular season and who implodes in the post season is a waste. But there are many other teams who DO need a guy like him during the regular season who would pay through the nose for him.
The key to any potential Cup victory for the Leafs lies not in keeping the core together but in breaking them. Marner can do more to help the Leafs win a Cup by being traded than by staying for another 8 years.