Todd McLellan is one of the most overrated coaches in the NHL. He has only ever coached teams with HOF #1 centers that are near the best in the league and never made it anywhere.
I still think the Oilers' struggles are more attributed to random variance than anything else, though. However, Chiarelli has been very bad. He traded Barzal for an asset that has already been liquidated into nothing and waived, he signed Lucic and Russell to awful contracts. He took a team with the best player in the game and another elite first line forward and tried to turn them into the 2011 Boston Bruins.
You have a Malkin/Crosby, Toews/Kane type duo. Why not try to build around them and mold the team after the teams that have 6 of the last 9 Cups, rather than a lucky team that has one of the last 6 (which can be mostly credited to random variance)?
He got bad value on the Larsson and the Eberle trades. The Barzal/Eberle combo that is lighting it up with the Islanders? He traded that away. He had that in his grasp. What did he get for it? Ryan Strome. To elaborate, he traded Barzal, Eberle, and Hall away for Ryan Strome, Adam Larsson, and a guy that they let go of in the expansion draft. Had he not traded away those three players, the Oilers would have quite easily the best top-6 in the NHL. McDavid, Eberle, Barzal, Hall, Draisaitl, and RNH.
Chiarelli traded away Tyler Seguin for scraps as the Boston GM, and he's at it again. I hope he stays with the Oilers forever and ruins McDavid's chance at a Cup.