You have some really good points here.
To add one more regarding the AFC East, the Fins were coming off a 1-15 year with yet another new head coach, which is the kiss of death, right? And NY made Brett a Jet, ostensibly to fill seats and sell T shirts & hats. And who would have thought that Geezer Favre would actually take a team heading downhill and make THAT much of an impact in the first new offense he's seen in, what, 47 seasons?
So you had NE without Brady, NY with an AARP QB that was getting tutored on the sidelines between plays and Miami with a new head coach and a new "noodle armed" quarterback who had to learn a new offense.
If this was EVER the year the Bills practically had the AFC East handed to them, this was it. At least on paper.
But as I used to say when I was teaching programming, "All programs work on paper".
I still think it was the high hopes after the 5-1 start that makes it hurt so much now. The Bills looked hungry, and focused and determined. They even engineered 4th quarter comebacks, though admittedly not against the likes of the Steelers or Giants. But, the higher you are on the ladder, the more it hurts when you fall off.
The sad (tragic really) fact is that no matter what the Bills have tried doing since the Levy and Kelly era, nothing has really worked to bring the Bills back to that level.