One way to think about it...roughly 100 years of hockey. To start with, in any ten-season-span, is Thornton a top-10 player? Let's clarify that the best 10-year-periods for Thornton are from 1999-2010, and 2005-2015 season.
For that first period, even though they played fewer games, I still take Crosby, Ovechkin, Malkin, and Forsberg over Thornton because of their utter dominance in 300+ games. Jagr is closer in games played and was a better player, and Sakic has the numerous playoff runs. Looking at defenses, Lidstrom, Niedermayer, and Pronger are hands-down more deserving than Thornton. In goal, I'd take Brodeur.
That's ten greater players right there. After that, there are still players who have a case against Thornton: Kovalchuk, Iginla, Datsyuk, Zetterberg, Leetch, Zubov, MacInnis, Roy, Hasek, Luongo, and Giguere to name a few. Personally, I'd still take Iginla and Roy over Thornton, if just barely.
Let us take 2006-2015. Again, Crosby, Malkin, and Ovechkin are easy choices. Datsyuk and Zetterberg should be ahead of Thornton for defensive and playoff play. Among defensemen, Lidstrom, Niedermayer, and Karlsson have to get the nod. In net, Lundqvist is just ahead of Thornton.
So here, Thornton might have a case. But still, you have players like Iginla, Getzlaf, Kopitar, Kane, Toews, St. Louis, Kovalchuk, Staal, Jagr, Subban, Doughty, Luongo, Quick, Thomas, and Fleury. On the strength of playoff performances, and without thinking much about it, I still take Kopitar, Kane, Toews, and Quick over Thornton.