Marner is a franchise player for the Leafs so would not want to agree to this trade.

Not trading Marner for that. Add 3 first round picks for consideration.

Every team and every coach and fan base in the NHL wants a player like Marner who can play on regular line, PP and PK and has tremendous leadership skills. The Leafs have been waiting over a generation to have a player like him and they are not trading him away as he will never be traded.

You are absolutely correct that this trade does not get the Leafs a return that upgrades on what they send away. But here's my pitch on why that's okay.
What do the Toronto Maple Leafs need? Defense, effort, and cap space. That's what this trade gets you. Find me (and I'm actually being serious here, not trying to be sarcastic) a team willing to take on Marner's contract and give up an elite defensive option on the level of Marner. I'm trying to be an adder if I'm the Maple Leafs and do the best I can with this situation. At the same time, this lights a fire under the team's behind. The only other way I can think to get some desperation out of this team would be to walk into the locker room and tell one of the big four "I'll see you in Detroit" because then they might be scared. That's the goal of this trade: to acquire defense, cap space, and fear.

I totally agree with your logic that one of the Leafs big four forwards has to be traded for a quality defenceman. I just do not think it should be Marner and think they should trade out Nylander or Matthews before Marner.

I just think that of all of the big four, Marner is the least untouchable. He is good, he is great, he is elite. But Tavares is the Captain, Matthews might win the Rocket and Nylander seems to be the only guy who gives a general crap. All of them have some level of untouchability, but I think Marner is the least untouchable of them. Of course that's just what I can see and I'm not in the locker room, so what do I know?

I noticed last night that Marner stepped up and took full responsibility for his part in the Leafs loss in an interview with the media after the game. These are the kind of players you want on your team as he really cares about the team. I actually think Nylander is the least untouchable player and probably explains why he was one of the only Leafs at a optional practice today as he wants to stay in Toronto but probably is worried about being traded.

That's a good point, and Nylander might just be getting traded, but I wasn't interested in speculation. I was interested in what I would do as Kyle Dubas, and I'm looking for actions that show me that these guys care, and Marner giving some interviews saying he didn't put in effort doesn't say nearly as much in my mind as William Nylander being the only active Leaf to face Dave Ayres in optional practice. It might be that Nylander going to practice means he's out, but to me, that's just a guy getting up and saying, "I've got work to do".