I debated internally the same thing. Muzzin brings balance to our D group.
The downside is his age. I don't want to spend a big UFA extension on a defensive Stanley Cup champion at 30+ years old who already plays a rough and tumble style on the left side.
On the flip side, the last time the Leafs had a right shot defenseman who could sit reasonably comfortably at the top of the depth chart was... Bryan McCabe. This is why Lou pulled the trigger on Zaitsev for term, because RD are so hard to find. Unfortunately, Zaitsev didn't work out the way we were using him here. Barrie, on the other hand, is Rielly on the other side of the ice and has played those top minutes. We have an opportunity to extend him let's say 6x7M in January and still be getting a lot of his good years left and give our weaker side more time to grow (or find new blood). He is already well aware that this team fits his playstyle, which shouldn't wear out as easily.