It really is a strange usage. But usually when you have an outlier on the edge like WJ, it's generally some young guy trying to cut his teeth. No matter the level or league. So that guy would be in their prime athletically, and you'd use them in a lot of ways.
For a guy like WJ to play this late into his athletic prime is extremely rare.
I think his step has diminished, but he can still turn in on. I really think the majority of his dip has come because he has been getting physically battered for a long time. After all, teams can double him and have figured out that if you out-physical him, he will coast. Some guys don't relish the physical punishing side of the game. Even at DL. Even at his best, WJ has been a terrible tackler in the back field.
From an offensive standpoint, I'd be happy to see any DE roaming on the edge looking for PKDs. I love DL getting organic PKDs, but their value isn't remotely comparable to a sack imo. Tips still gets caught sometimes, but he is very seldom batting balls up to get picked. A PKD on 2nd down means you're punting unless it's a very short distance to the first. But even sacks on first down that get converted bring tremendous value. When you stack up physical hits on a QB, you drastically change the game.
Ages ago, Football Outsiders I believe it was, had an article on QBR after hits/sacks and hurries. As you would expect, QBs that took 5-10 QB hits/sacks/hurries in a game saw drastic dips in production. Hurries don't have as much impact, especially without the hits, but you change the game when you disrupt the QB. PKDs slow the drive but don't have that compounding.