I don't see that we need Clowney, Griffen, Yannick or anyone. We had Clowney last year. He drew double teams and we had a bottom feeder pass rush. Aaron Donald would draw double teams on this team and we would have a bottom feeder pass rush.
Pete's D requires a run stuffer/pocket collapser like Mebane/Reed and then three guys that can win 1:1 to get sacks.
Clowney is a more athletic Red Bryant; stout against the run and an outside pocket collapser, but not slippery enough to beat blocks and get to the QB. Maybe Irvin has learned some pass rushing skills in his absence, but again he's a guy that relied on his physical gifts rather than any technique to get sacks.
I think this D is just stuck in 2nd gear all the time because we have a bunch of good athletes; but not anyone besides Bobby, KJ, Kendricks, McDougall and Diggs that know how to play their position.
Bottom line: if Clowney stays or walks, everyone on that defense is going to have to elevate their game if we're going to have an elite defense again. The problem is we're not seeing the lights come on for the young guys fast enough and Diggs is the only one of the 5 I mentioned that will be under 30 this year.