Part 1, did I ever say any of that?
Part 2, Toews and Kane also have played on better teams than Thornton and Marleau have. They also play vs weaker comp, more Kane than Toews. That's no excuse for the difference, but it is fact.
Marleau and Thornton have also come clutch in big games, but clutch opportunities only come with the help of teammates and when you have superior teammates, as Kane and Toews do, they get more opportunities to be clutch.
So yes, probably, those Chicago teams were better than any teams that Thornton or Marleau have been on. Marleau and Thornton certainly never had a Keith on the backend...and Toews has the dependable Kane, while Thornton and Marleau just have each other!
But, there have been SC-winners in recent winners/finalists where the stars had a "worst" supporting cast than the SJ duo. What Thornton and Marleau have achieved, a combine 3 WCF wins in 10 years...that just is not worthy of the quality of rosters they've had.
Lastly, and this is subjectively speaking, Thornton and Marleau typically don't do justice to their capabilities come the playoffs. Even if you can criticize their supporting cast, both players, visually, aren't doing all they can to maximize their team's chances to win. Look at the team's constant struggles on the PP. Look at Marleau frequently being unable to solve a hot goalie. Look at Thornton forcing passes that are intercepted.
The Sharks's failures are in the context of Marleau and Thornton not playing well. Even if you want to say the issues go beyond them, the issue still start with them. IF your core is rotten, nothing will be gained by tinkering with the fringe.