Keefes coached this team into good spots. When you look at the issues you see
1. Unforced errors in good positions
2. Lack of finish
Neither of those issues fall on him. He can't make Marner ice it instead of flip it. He can't waive a wand and have Dermott handle the puck like a sane person (and he did all game) instead of overskating the OTG.
We've lost games on simple bad decisions and lack of finish. It's really that simple.
I'd love to hear a candid rebuttal from a real NHL coach about the idea of puck possession as part of the Leafs progressive thinking. From what I see as a fan, I think the constant regrouping and bumping back has a number of issues which aren't really discussed at a tactical level and is usually taken for granted as a smart move without any critical breakdown. Here's my amateur fan attempt:
1) The Dermott Game 6 error shows us not every player is good enough in every situation to turn back with the puck and retreat into higher danger areas when safer outlets are available. This isn't the only time, but I can recall a number of occasions when a Pierre Engvall might curl back in the neutral zone when he can just flip it in and retrieve. The desire to create a play by bumping back is also part the Galchenyuk blunder. He could have taken the hit or just shoveled it in deep to initiate a puck pursuit, but got fancy trying to connect with Muzzin like they're Lidstrom Ralfalski back there.
2) Moving the puck backwards kills momentum and creates higher danger situations when there are turnovers.
In scenario a, the forward puck carrier gathers a ton of steam and skates up the ice with the puck, either to make a play upon zone entry by skating it in, pass it off and also pushing the defense backwards. Think about Nathan Mackinnon torpedoing in.
In scenario b, the forward can flip it in and have numbers to support a puck retrieval in the corner or along the boards. Think Ryan O'Rielly.
In scenario c in Toronto's case, the primary puck carrier skates full steam ahead only slow down in traffic and drops the puck back down the ice and waits for the next puck carrier. By the time this regrouping happens a few times, the guy carrying the puck is looking at a massive traffic jam of 9 players loaded up on either side of the blueline and has to thread the needle for an entry. He also has no one back to support if there's turnover. Wouldn't that make you nervous as the last man back and the rusher?
A lot of these tactics are taken as new age dogma, but they don't really work in the field all that often.
Also, his spamming random video game line combinations looks less like Nick Nurse and more like Xbox.