One of the best players in the league, and the 2nd best player on the team.
Having good defense and depth does not require throwing away our best players. We've already done it, through a much more difficult league and team situation.
And so far, Treliving hasn't shown an ability to improve either, or effectively use cap space.
The only cap space that is relevant to Marner is the cap space he takes up, and we should be able to improve our defense and depth with the cap space we already have.
Saying you want to upgrade the defense is just a generic statement of want. It isn't a plan of how to actually do it successfully, or an explanation of why it requires removing one of our best players to accomplish.