As seventies has already noted, these arguments cut both ways. Top line guys typically have fewer checking responsibilities, but they also face greater checking pressure than checkingline guys, and guys like Gainey actually had really good linemates most of the time (Gainey often skated with Lemaire + some other good winger like Cournoyer).
Some of it also comes down to looking at the linemates players are getting here in the ATD, and comparing those guys to the linemates they had in real life. Specific team context counts for a lot.
Backstrom I get, but I have no idea why you'd single out Benn and Getzlaf for scrutiny vis-á-vis linemates. Throughout the course of his prime, Getzlaf has generally been the best scorer on his lines. This is not true for every single season, but on the whole it is definitely the case. Same for Benn, who has outscored the famous center you're thinking of in Dallas (overall, and at even strength), and has often skated on a line with another famous center who is now old and not that good (stupid undrafteds).
The guys who should come in for the most scrutiny in these numbers are those who were clearly not the best offensive players on their own lines for major parts of their primes. That list includes Gainey, N. Backstrom, Provost, Lemaire (??), Mullen, Hodge, S. Smith, Nevin, Marleau, Armstrong, Martin, Ellis, C. Lemieux, Tikkanen, Mosdell, Bourne and Lehtinen...just off the top of my head.
If we're going to criticize these numbers based on linemates, guys who lead the attack like Benn and Getzlaf are not going to be first against the wall.