Biggest problem is Dubas is known for being an advanced statistics junkie.
This isn't to take away from the fact that he OBVIOUSLY knows the game, and knows the value of players; but would Dubas value players like Weber, Suter, Burns or Byfuglien the same way he would value a player like Josi, Karlsson or Pietrangelo?
The sad truth is regardless of Tavares's fate, the Leafs need to address that blue line, especially on the right side; and the only teams that would likely be willing to move a top 4 RD right now is Nashville, Montreal or Calgary (Subban, Weber [ironically], or Hamilton, respectively).
Does Nylander get Subban or Hamilton? Absolutely not.
Could Nylander get Weber? Probably, but even then it's close despite age/contract differences because 1. we don't know what Nylander is demanding and 2. he's a winger and they need a big minute 1D.
I don't think Tavares works in the long term for Toronto just due to cap issues; they could make it work by moving Nylander, but there's no chance that team can stay afloat with Marner likely to be making 8-10m, as well as Tavares and Matthews both taking 12.
The Leafs just simply don't need to add any offensive firepower to their arsenal, they have so much of it in abundance that they need to address their gaping weakness in their own end.