I think the biggest problem is that 1) we don't have the personnel to cover effectively and that 2) "coverage" isn't what it used to be, Ty Law couldnt play in today's NFL with the rules/officiating the way it is.
If you give a good QB enough time he will get the ball downfield, you can't cover forever, you NEED PRESSURE!
People will talk a up a storm about how 'it's now a passing league, blah, blah,, blah...' and will also talk about how 'the QB position has never been as critical as it is now', that tells me that putting the QB on his back is more important than ever.
If you can't get a damn pass-rush then it's simply a battle of which offense can make the fewest mistakes, I don't want to rely on my enemy to make an error which I might capitalize on, pressuring the damn QB has the potential to 1) mess with his body, which makes him less effective, and to 2) mess with his mind.
I watched in horror how the Giant's pass rush EVISERATED what was the best passing offense the league had ever seen, I felt similarly when the Lion's D-line gang-raped Brady and shockingly we never got into an offensive groove.
Yes, the secondary sucks right now and I thought getting rid of Sanders was stupid, but we also can't get any damn pressure.