He's playing the same average ice time as defenders like Seider, Josi, Heiskanen. He's 9th in the league for ice time by D. He's not in that tier of player obviously but this is what happens when bad teams have to put guys in positions that are above their paygrade.
Other GMs aren't as dumb as the fans, he still has trade value. He's been terrible lately but he's playing vs other top lines which he shouldn't be and playing way too many minutes. If he get's put into a team where he can play sheltered minutes on a 2nd pair he will look better. He's only 31 and still an elite skater.
He has no place on this team going forward though. Now that Lane has proven himself to be an elite puck moving defenseman at the NHL level, you can move on from Matheson and surround Lane with big stay at home guys. Still, Matheson played valuable mintutes the past 3 seasons for the team and played in way over his head for his entire stay here. Playing number 1 minutes on a rebuilding team for a guy making under 5 million dollars, I don't know what people were expecting.