My theory these days is this:
For 5 years we've thought the goalies were an issue. Several goalies were auditioned, all had their sv% plummet.
For 5 years we've thought the defense was ultra weak. Oodles of dmen have been auditioned, all have seen their +/- and GA60 plummet.
I've concluded that the 'core' has an attitude problem and are refusing to consistently play within a system that will let the goalies and dmen do their jobs as they should.
I'd even go so far as to say Schultz has been hung out to dry to some degree because the forwards around him are inept.
Specifically, I believe that the 3rd line has been this way, and then rotating individual culprits on the other lines.
It's like a big chess game. You see the dmen and goalies getting hung out to dry, but it was the move 3 steps earlier that created the check-mate situation.
TL;DR The forwards defensive acumen is to blame for all of this.
I think you are using some faulty logic there. Look at who we've had on defence and where they've been forced to play here. Look at where they end up once they leave here. It's always lower down the depth chart than we were playing them with much better defencemen ahead of and around them to cushion them.
Petry left here as one of our best defencemen. He's second pair in Montreal with at least two better dmen on the team.
Smid left here after being considered one of our best guys and playing top pair for a long time. He's barely 3rd pair in Calgary.
Gilbert was a top guy here for a bit , third pair after he left here.
Marincin people were amazed at and thought he'd be in our top 4, now barely cracks the Leafs roster.
On and on it goes. We overrate what we have on defence, including Shultz, and bring in old overrated players to replace them i.e. Ference and Nikitin.
It amazes me that anyone thinks our problems up front are even worth considering before we fix our d.