Nylander has better raw tools than Marner, and always has. If Nylander thought and saw the game at Marner's level, he'd be in the Hart discussion. I get the frustration with this team, and Marner. But, I'd trade Nylander over Marner, because at the end of the day, it boils down to this for me. How many guys in the NHL see and process the game at the speed Marner does? He is in the discussion for the smartest player in the league. You just don't give up on guys like that. While his contract is far from ideal, I feel moving Marner is likely to end up in the long-list of big roster blunders by the Leafs such trading the Big M, trading a first rounder Tom Kuvers (not only a pick that became Niedermayer, but also could have been used as ammunition to get Lindros), trading Rask for Raycroft, waiving Steve Sullivan, etc.
Do I like Willy as a player? Absolutely. But, I don't feel moving him could burn us the same way. It's fine for the fanbase to disagree, but I'm always going to gamble on the guy who can think the game like Marner unless you are talking about guys with truly freakish tools while also having high-iq's (Matthews's shot, McDavid and MacKinnon's skating, etc).
A lot of the smaller guys with insanely high-iq didn't really explode until they were older than Mitch is currently, and a bit more physically developed. Kane had his career year at 27, St. Louis at 29, Marc Savard at 28 (although rule changes contributed). The exception is Giroux at 23.
I'll always be a Leaf fan, but I would find it devastating to move on from Marner. It's just impossible to find players who think the game at the level he does. What he's accomplished without one other skill you can point to, and say he's clearly top 20 in the league at (unless you point to vision/passing as different than IQ) is astonishing. Even with the cap-hit disparity, I just don't see Nylander's top-end being comparable to the peak years out of Marner.