As a Leafs fan, I would definitely not deal Marner. Kid has insane skill and great hustle, a very rare combination. Re: the Kane comparisons, he's not quite there skill wise, but isn't too far off - he's one of the most talented smaller wingers we've seen since Kane. Also has an edge on young Kane in terms of hustle and 2-way play. I definitely think he has "franchise winger" potential, and if Nylander moves back to centre, Marner will be our only truly high end young winger.
Having 3+ elite forwards makes a team damn hard to handle - just look at Crosby/Malkin/Kessel and Toews/Kane/Hossa. If we dealt Marner, and our forwards became more "Matthews, Nylander and supporting players", I think we become much easier to stop offensively. We don't need to make any crazy moves for a mega star dman, we just need a solid 2D to complement the 1, 3 and 4 Ds that we already have (Rielly, Gardiner, Zaitsev). We need a Tanev type, and we can and should get a player like that without giving up any of Matthews/Marner/Nylander. Pieces like Kapanen, JvR, 1st round picks, etc. can be on the table, but not Matthews/Marner/Nylander, these 3 are the core of our team going forward.