Show me one contender that would play Roussel in the top nine. Do you really think he would play in the top nine on Washington, Pittsburgh, Toronto, Tampa, Nashville, Winnipeg, Calgary or San Jose?
I can't "show you" when that player isn't on that team you know.
But you seem to be under the impression that those "contenders" you mention have amazing forward depth. In most cases, they don't. But let's do the exercise and assuming a healthy lineup.
Washington. Let's assume Ovechkin, Oshie, Wilson, and Vrana are locks on the wings. Who does Roussel have to beat out? Burakovsky who has been a healthy scratch this season? Stephenson? Boyd? Smith-Pelly? Roussel on the left and Connolly on the right looks good to me.
Pittsburgh. Assuming Crosby and Malkin C different lines with Brassard on the 3rd, you have Kessel, Hornqvist (if healthy), and Rust on the right. Guentzel and then Pearson? on the left? The next batch is Sheahan, Simon, Aston-Reese. I think Roussel can play ahead of those guys.
Toronto I would say maybe not, but it's not like the Leafs' left side is all that imposing. The Leafs' wingers are Marner, Hyman, Nylander, Marleau, Kapanen, Johnson, Lindholm, Brown. At the end of the day Roussel is a veteran that a coach like Babcock knows what he's going to get. Johnson or Roussel on the 3rd line come playoff time? I don't see this as a slam dunk.
Tampa. I would say probably not when Miller comes back. But right now their 3rd line winger is Erne. Not a hard guy to beat out come playoff time.
Nashville. I would say maybe not, but this is really not a team with exceptional winger depth. But I think Jarnkrok is proven and usually offers a bit more offense than Roussel so I can see Roussel spending most of his time on the 4th line come playoff time on that team.
Winnipeg. This is a tossup. The Jets are deep on left wing. Right side? Not so much. Wheeler is a lock. Roslovic, and Tanev? If Maurice is ok with playing Roussel on the right I can see him earning a spot over one of those two.
Calgary. They have Tkachuk, Lindholm, Gaudreau own the wings. The others are Frolik, Bennett, and Neal. Those 3 are not playing well. Natural position wise it's Bennett vs Roussel. It's really not that hard for Roussel to earn a 3rd line spot on that team.
San Jose. Hard to say but let's just say probably not. Their wingers are producing offensively.
Your turn. Why don't you "show us" why Roussel wouldn't come close to playing on the 3rd line on those teams?