Much like 'I'm happy with the season if...' a major factor is doing or not doing enough is expectation.
If we offered Bills fans the chance to lock in 36 points before the game, I think a bunch of them would be fine with it, especially since that would have handily won the regular season game. The fact that KC scored more makes me feel the Bills faithful are more upset with their D. And, given the screwed-up OT rules, they probably feel they could have at least kept the game going if their O at least had a chance.
Along the same lines, if I had offered you the chance to have SF end with 13 (and AR playing the whole game), and/or the chance to go into the 4th quarter with 3, wouldn't you have been willing to accept that with confidence? Hence, I'd hang it on the O.