Quoting: vr1995
the season starts in 2 months, while I think there's more value in keeping him because he's nowhere near as bad as everyone says he is, he's still not a negative value
Here in one sentence is the root of the problem from which most Toronto fans suffer: the conflation of trade value with playing value, and the conflation of both with some "Platonic ideal" of universal value.
On the last point first, players don't have the same value to all teams. Say that a player is a perfectly good #2C. That player would be worth more to Boston, who are pretty anxious to get a #2C, than he would be to Edmonton or Toronto, who are rumored to have 2 pretty decent centermen already.
To my knowledge, nobody who says Muzzin is a negative (trade) value, or even low (trade) value, is doing so based on the argument that he's a lousy defender or even "bad" in general. On the contrary -- many of us who decline or object to Muzzin trades to our favorite teams do so on the basis that his cap hit does not justify his acquisition, especially if we're talking about non-contending teams like Anaheim who won't sniff the playoffs this season or contending teams like Los Angeles for whom Muzzin is a numerical impossibility. I have made the argument in numerous posts that Muzzin has significant value to the Maple Leafs because taking him out of your defense would be to seriously weaken it -- more value to Toronto on the ice than losing his cap hit would recover, because you have no real internal replacement for him (please don't try to tell us that Sandin or Giordano is) and what's left on the free-agent market isn't even as good as your own guys. (DeHaan? DeKeyser? Please.)
You're absolutely right that Toronto should keep him. He's worth more to the Maple Leafs than he is to any one of the other 31 teams. The defenses of most contenders don't need him because their top-4's are mostly well-set and the defenses of all non-contenders don't need him because HE WON'T HELP.