The one thing we fans tend to forget about Clemons is that he was coming off of injury. He actually missed some time early in the season because he wasn't all the way ready to be back.
The thing about ACL's is that it takes a year to get ALL the way back. I personally theorize that DE is even more than a lot of positions when it comes to an ACL; there is a tremendous amount of lateral pressure on your knees when you explode off the snap, run full bore at an OT, then lean into him to try to get the corner. That's a ton of lateral pressure on your knees and it takes time....not only for your knee to get all the way back, but for you to mentally trust it to be there.
We saw in the playoffs, against the better competition, that Clemons regained his form from last year. He's also the best LEO we have against the run. What I mean is, there are guys that can play LEO and play the run better than Clem, but they won't rush the passer as well. There are guys like Schofield and Mayowa who can probably rush the passer as well as Clem, or close to it, but won't play the run as well.
The other thing I've noticed is that the Seahawks want to be able to show their players that they get taken care of. Look at Miller as well as Clemons: they've both only ever done what was asked of them, and they got rewarded. We might not understand it, but Miller earns his contract in a lot of ways, not just catching the ball. Clem as well. Rice and Bryant didn't have good value for their contracts. I know it's a business, and I'm not saying Clemons won't get cut, but I think the Seahawks try to keep guys that have been rewarded as long as they can. What good is getting your contract that you've earned, only to get cut 2 years later when that salaray is inconvenient? Players will be more likely to take a more team friendly (yet fair) contract if they know they'll earn it all. I think the Hawks keep Clemons unless they absolutely can't. He still has value, he looks to be earning his contract, and the Hawks should have the cap space now.