Muzzin wasn't a top pair dman in Toronto ever, always playing behind Morgan Rielly and always on the 2nd pairing. He might have been one in his Stanley Cup days with LA, but then he was paired with Drew Doughty a Norris & Conn Smythe Dman, but that was nearly 10 years ago now.
Last year Jake Muzzin played in only 47 games (missing almost 1/2 a season) produced 3 goals 14 points and was a -6 +/- on the year. Those are clearly not top pairing numbers, and even questionable 2nd pairing to be honest. The best ability for an NHL team is availability, because without that you can't have reliability.
The previous 2 season prior he played in 53 games each. Injury and age and due to his style of play and now mounting concussions, even as 2nd pairing player he is no longer reliable,. He is closer to falling into a bottom pairing player then he is close to playing as a top pairing. Years on his style of play take a large toll on a player's body and his NHL longevity, and his career is slowly nearing its end soon.
At $5.65 mil he can not longer validate his cap hit contribution, and I think a lot of Leaf fans would be comfortable moving on from Jake, thank him for his warrior service, and use that cap space on a younger, better and healthier player.