I tend to look at it two ways. I partly agree with
@The Panther that it's player personnel. I partly agree with
@Del Preston that it's been coaching/the wrong hires.
Mclellan and Hitchcock have failed here due to the roster makeup. Hitchcock, arguably, has a slightly worse roster to deal with than Mclellan. That said, they (along with Quinn) are both from the "old school"/"dinosaur" approach to coaching with a majority of millennial generation players on today's rosters.
Hitchcock nailed it in his After Hours interview a month or so back when he gave an explanation on how he's had to change his coaching style over the past 20 years to get player buy-in. He alluded when he first got into coaching at the pro level in the early 90's, the players were told "This what I want you to do", and they went out and did it. Then the generation changed to "This is what I want you to do and this is why" around the early part of 2000 and the players went out and did it. Then around 2010 or a little after, he said the generation of players changed again to "This is what I want you to do, this is why and this is what's in it for you". He said to be successful this day in age, you need to cover all three of those things: the what, the why and the reward. As we're approaching 2020, is their a 4th element coaches need to incorporate now? Possibly.
I don't think J.Q. is going to be the answer either. This roster isn't going to perform any better under him than it does Hitchcock. He's also going to want big dollars and term so if he flops the way Hitchcock has done, then what? Hitchcock, over his career has been a pretty damn successful coach but his style just isn't working. If McDavid has any say (this is getting into Jack Eichel territory where it was rumored he got Bylsma fired), I could see them trying their hand with a younger hire and Kris Knoblauch being named the Oilers HC...risky move being his first HC gig at the NHL level, but he's young enough (40) to find the right balance between the younger players (5-6 years and less of NHL experience) and the veterans (6+ years of NHL service).
Todd Nelson would be another good choice but with who is still in charge and those same people passing him over not once, but twice, I doubt he even applies.