There are lots of reasons to be concerned, but I'm generally pretty hopeful about the defense. In context, last year seems more like an anomaly than a trend. In the two previous years (under the much-reviled Ken Norton, Jr.), the defense finished 12th and 15th (and 5th against the run). The chances that Pete fixes the defense after seeing what didn't work last year are good.
And Pete did a lot this off season to address the defense. We were very active in the free agent market, getting Dre'mont Jones, Bobby Wagner, Julian Love and Jarran Reed. We also got some nice pieces in the draft--Witherspoon, Derick Hall, Cameron Young and Mike Morris. And then there was addition by subtraction. If Pete and his staff did a good job of self-scouting, we'll be better for it. (One interesting case in point is Ryan Neal. As a strong safety, he just didn't do enough in run support. Despite his high rating by PFF, Neal had a total of 66 tackles as opposed to Julian Love's 124. Neal didn't play in as many games, but only averaged 4.7 tackles per game compared to Love's 7.8.)
Yes, we'd all love to have that huge nose tackle for this defense. I wish we had Vita Vea on the defensive line. But there aren't enough great nose tackles to go around, unfortunately. But if that ends up being the only deficiency on this defense, I think we'll survive.
The Chiefs went from 27th to 16th last year. And that was after losing Charvarious Ward, Tyrann Mathieu and Jarran Reed. I like our chances for seeing a significant improvement.