I'm surprised so many Leafs fans are against this deal. The way I see it Zacha and Nylander are about equivalent as prospects and McLeod >>> Marincin.
Most of it has to do with not needing to trade Nylander to begin with while the rest has something to do with having even fewer NHL-quality defensemen, especially for 17/18 like OP is suggesting.
If Marincin can carry the form he held in the second half of last season into this season (and by extension, into 17/18, as that is the earliest year in question), then the Leafs might as well hold onto him for that long and have another defenseman with experience rather than throwing in yet another rookie into the mix like Valiev, Loov, Dermott, or Nielsen.
And that's not even bringing up Mike McLeod getting decent ice time as a rookie in place of Kadri or Bozak or whoever on the third line (all of whom have NHL experience which, if the Leafs are in and around the playoff picture, would be nice to have).
It's just a trade that doesn't need to be done. That's why you have "so many Leafs fans" against the deal.