I like the point you made, but you still don't account for offensive ineptitude. How many times last year the Bills' offense gave up the ball or lost yardage late in the game. After being on the field for so long, it's not really surprising that a tired defense would get a bit deflated and give up the points.
Our defense bailed us out more times than any reasonable fan should ask for. Now it's up to our offense to stay on the field and keep them from getting tired. Once that happens the fourth quarter shutouts will be a more common occurence.