Who or what is the blame for the regression?

You're right.  It's more like we were a very mediocre team who supposedly added much talent and would have the benefit of better play from second year guys who have become worse than bad.  Pathetic is about right.

And the **** this is about QB'ing it's a far deeper than that.  You don't need a gunslinger with an arm like Rodgers or Cutler to win but you do need a HC and an OC who are sharp enough to scheme and play call to win with who you have.  Christ by the end of the 2015 season Manning could barely throw a ball 20 yards on the fly and Denver won a SB with him.

If you want to get to the bottom of it treat the root cause not surface symptoms or this team will never be healthy again.  Here's the problems.

"Offensively, teams trained to play not to lose typically do. These Bears are Fox's creation, an overly cautious bunch worried more about protecting the football than advancing it, a group of players more concerned with not making a mistake than making a big play."

"But understand the cost of that safe approach in Jay Cutler's absence. On third-and-16 on the Bears' final drive, for instance, Hoyer hit Cam Meredith on a 6-yard hitch route. Efficient? Sure. Intelligent? Not if you want to keep the chains moving to get in field-goal range."

"Too many examples of that conservative mindset exist to think the Bears are better without Cutler."

"Alarmingly, the offense also has shown either an unwillingness or inability to adjust. Either tendency indicts the coaching staff. When Jeffery catches six passes in the first half for 90 yards and the Jaguars hold him to one catch for 3 yards in the second half, it suggests the Bears have been outsmarted."

And none of this is so much vote for Cutler as it is a vote against Fox and his insane personnel decisions and continued insistence on playing a brand of football no team without an extremely dominant defense has been able to win with in over 20 years.  He wants to play 1990's football in 2016. 

You couldn't bring in Tom Brady or Aaron Rodger and win in this system he wants to run.

For all of the injuries he's faced and the rookies and second line players he's had to play with Fangio has kept us in most games and we've yet to have been blown out that way.  But offensively.  Yeah, there we have been blown out.  How many NFL teams who roll up 400-500 yards in a game can't manage more than a single TD and end up leaving points on the field settling for chip shot FGs?

You want a better team?  Get a better Head Coach!