If they where close to being average......we would have won 10-11 games last year. You are asking the pass rush to be absurdly dominant to survive.
There are a lot of reasons we didn't win 9 games last year. Singling DB out and trying to justify sacrificing value at a need position (DT) for a DB that may not be as high on our board with blanket statements like how many games we'd have won if we had Reggie Nelson and Revis back there isn't going to improve us much. I do think there's a lot of room to improve in the secondary, but the opportunity will present itself, and when it does Gettleman will tackle it. He was handed a team with 2 early round draft picks this year and a negative cap figure in the double digits. I think he's made tremendous improvements in the roster overall given what he inherited so far.