Worst Mistake of the Shanahan Era

[HailGreen28]

I've said we had a "top 10" defense before Shanny and Haslett. Looked it up on nfl.com (example link):

2013(so far): #28 yds/g, #31 pts/g.
2012: #28 yds/g, #22 pts/g.
2011: #13 yds/g, #21 pts/g.
2010: #31 yds/g, #21 pts/g.
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2009: #23 yds/g, #18 pts/g.
2008: #4 yds/g, #6 pts/g.
2007: #8 yds/g, #11 pts/g.
2006: #31 yds/g, #27 pts/g.
2005: #9 yds/g, #9 pts/g.

It's a bit rosy colored to suggest we had a sure top 10 D the past few years, but changing our D is still the worst mistake Shanny's made. How much of this blame is the switch to the 3-4, and how much is Haslett, I dunno.