You've got the whole thing exactly backwards.
Ultimately, in the win/loss column, raw production out of context doesn't matter. At all. You don't win a game by scoring a lot of goals. You win a game by scoring more than you give up. Scoring line, checking line, fourth line, first line doesn't make a difference. Every line either wins its minutes, stalemates or loses them. While it's self-evident that teams should seek to win minutes with a first line, that does not change that stalemating is far preferable to losing them.
If a Nail Yakupov line scores 40 goals on the season, while giving up north of 70, not only is it not helping the Oilers, it is killing them. You might win a half-dozen more games if you just sent out three guys who could play 15 minutes a night to a tie.
Kunitz is stalemating. That's not what you want. Ideally you reduce his minutes in favor of someone that has a better chance of winning his minutes. It is not clear that we have that in even 1 left wing right now, let alone 2.
Plotnikov, so far, has a 2 not-good games, 1 good game, 1 benched game, and 1 game where Liam Neeson was killing his shady ass countrymen. In a 4 game career, that's not alarming, but there's nothing about what he's done that says "get him in there for 20 minutes against the other teams' best D." In fact, his game only improved when we got him away from the other teams' best D.
Sprong isn't going to go on the first line. I'd like to see him tried (not that I know it would work), but it's not happening, unfortunately, so there's no point griping about it.
If Eric Fehr comes back and looks even half way decent in his first game without hurting himself, there's a case to be made to sub him in for the second game. A good case. We already know he can finish. We already know he can play defense. It stands to reason he could score more than Kunitz is without hurting you the other direction even if he has zero chemistry with the other two. Same with Dupuis. We know he can play defense. He can't finish, but he gets to so many danger spots that pucks go in on volume shooting. They're both practicing, so it's only a matter of time until they're back.
Unfortunately, they're not cleared yet, so short of putting Cullen on the first line and letting Rust and friends get crushed worse than any marginal improvement you might see on line 1, there's little to nothing that can be done about Kunitz at present.
The guy isn't even skating, let alone practicing and there's no timetable for his return. How are we sitting here asking for him to be subbed in for some other guy? He's on freaking IR.