D Law is called on to do, and does do, the dirty work inside. If he is double teamed, or the plays are run to the opposite side of the field, he's obviously not going to have the sacks and tackles to "justify his contract". He forces running backs inside, applies enough pressure on the quarterback to keep him in the pocket, etc.
Yes he got paid based on the 14.5 sack season, but a lot of players get more sacks in one season than any other, Ware had a 20.5 sack season, but not every season. The NFL is more complicated than that. And remember he did have shoulder surgery in an off season, that was bothering him most of the season before that.
The sacks aren't coming in part because teams can run inside on Dallas all day, besides being able to throw quick passes because the DBs and LBs (like Smith) don't cover well.
Is he a top-5 DE? No, and is somewhat overpaid, but overall he's a very good DE, he does more than enough to justify keeping him on the team past this year. It's the same with any player, if you play with a team that has a lot of other good players, you'll do better than being on a team with a poor supporting cast, which is the case with Lawrence currently...