I look at it this way
Rodgers has had some ups and downs in his numbers. For example, his TD to int ratio was actually worse in 2010 than it was in 2009.
Also, seems like people want to start the Rodgers decline talk at the 4-year period from 2016 on out in order to control the narrative while purposefully leaving 2015 out. That year his TD total dropped by 7 from 2014, his pass yards by more than 500, and his completion % by 5 points.
My guess is that's because his numbers jumped back up again next year after he started the run the table thing. Went back up to 40 TDs and 4,400 plus yards, and the 104 rating.
Now, it's true the normal rating and the QBR both dropped from 2016 to 2017, but they were both still better than 2015. Imo, you can't judge accurately whether he was in total decline from 2016 through the short period of 2017 because he never got to finish that year. Plus he was on a TD pass tear in the two games leading up to the Barr incident.
The only seasons I point to as a decline barometer are 2018 and 2019. But the reason I didn't sound off the alarm bells over that was because Favre had a similar thing happen in the two-year period from the year Sherman got fired and McCarthy taking over. His numbers were pretty bad there, but then jumped up again in McCarthy's second year kind of like Rodgers's did last year.
https://www.pro-football-reference.com/players/F/FavrBr00.htm
Guess all that to say coaches going bad, new systems having to be installed, all of those can have a correlation to a player declining or getting resurrected.