From the conversations I've had, Marner wants to play out his contract with the Toronto Maple Leafs. He's got one year left. That doesn't mean he's going to leave after next season. Of course, there's a risk since he'd become a free agent, but Marner negotiated his deal, including the no-move clause, in good faith.
And are we sure Toronto doesn't want to extend Mitch? I think Treliving would rather extend than trade, but those discussions haven't started yet. It's still early days.