You can't keep a defensive coordinator who can't do his job for three years and expect to have a great defense.
That's true to an extent. However, I'd argue that because it takes so much longer to form a good defense, you might not know how good he really is.
We've made so many DC changes over the years and none of them have worked out. I'm getting to the point where I feel like that's not a coincidence. We need to stick with one for at least 5 years to see major improvement. Defense takes a much longer time to get to be a dominant defense, compared to an offense.