I think the issue is the entire body of work, not just this year. This is his 5th year as DC as his defense statistically is no better than what Rob Ryan got out of his D. So people as the question, if Ryan was a scrub and was fired for the poor performance of his D, why is Rod not held to the same standard?
Of course that begs the question if Ryan was fired for poor performance of the D or was it a personality clash with Garrett, Ryan was getting too much of the attention by the media and such. And if people are hired and fired in this organization based on personality and "culture fit" and not based on their ability to actually do the job, that might be part of the problem right there.
The other issue is that Garrett is the head coach and has had 8 years to fix the defense and has failed miserably. Romo never had top ranked defenses year in and year out even though he played at a probowl level many years and now the same thing is happening to Dak, putting up 30 points a game and still losing. So people start asking the question, why has Garrett in 8 years not fixed the defense? Isn't the head coach responsible for the performance of the entire team, including the defense?