Leafs will trade one of their top forwards just to do "something", but that's not what their problem is. It'd be like Oilers trading McDavid or Draisaitl. Though at least the Leafs might get a good player back, but if not then it's just making the team straight up worse.
IMO if the Leafs were to approach, say, the Arizona Coyotes and ask for Lawson Crouse, Mattias Maccelli, and Sean Durzi I believe it could make them a much deeper squad:
Knies - Matthews - Crouse
Maccelli - Tavares - Nylander
Cowan - Kampf - Jarnkrok
McMann - Holmberg - UFA
Reilly - Durzi
McCabe - UFA
Benoit - Timmins
Woll
UFA
They sign Durzi to something like 6M AAV and they still have almost 14M to upgrade the roster with a 2nd pair RHD, a top-nine forward to push Kampf down to the 4th line, etc.
So while it might be too explosive of a move, I think moving Marner has the potential to make Toronto a better overall team. We talk about moving Mikko hypothetically to keep the depth but knowing the Avs they will re-sign Mikko and try to fix depth at the TDL.
Toronto, though, has a unique opportunity to improve the rest of their roster while keeping three of their core-four up front in prominent roles and allowing for the internal growth of Knies and Cowan.