A poor trade is better than no trade.
Getting something tangible, even if it's in the form of draft pick capital in future trades, is better than losing a player for nothing.
We've also seen Tampa walk their captain in Stamkos, right to the brink. not once but twice (this summer, and in 2016). However high people value Marner, he's simply not as successful, or important to the organization, as Stamkos is for Tampa Bay - and if they can walk him to the edge with their heels dug into the sand, there's absolutely 0 reason why the Leafs can't do the same, even if that means letting Marner walk.
And Woll's extension.
There is no "extra Tavares money" - it's either already been spent, or will be to keep Knies and McCabe.