I'm sure the fans don't care, but I'm sure the players will, and I'm not just talking about Nylander. There are long term consequences to mistreating players. For example, during the trade deadline a few years ago Mtl wanted Jannik Hansen. Hansen is good friends with Lars Eller, so Hansen asked Eller about Mtl. Well, Eller didn't think he was treated well there, so Hansen nixed any possible trade there.
The hockey world can be a small place.
It's public knowledge that a promise was made. If that promise is broken, no player will take Dubas' word on anything ever again. From listening to players talk, my impression is there are promises and handshake deals made behind closed doors about many different things from opportunities to earn more ice time to being moved along if things don't work out like Dubas did with Leivo. It's not just what's written down on the contract that matters.
What player wants to sign with an organization that they think will screw them over at the earliest convenience? Good luck with those UFAs.
And the expectation isn't that Nylander won't be traded for the length of the contract. It's that he won't be traded as long as Dubas is GM. There's a difference. If Dubas is fired, that means the team isn't doing well, and if that happens the Leafs are free to make changes as they see fit.
As for not making promises, isn't this about the same as giving a player a NTC. If Dubas didn't make that promise, I don't think Nylander signs for the numbers he did without a NTC, and Leafs would be worse off this season for it.