He was never a fast skater, but was just fast enough to get where he needed to be to score the goals or defend. He then had a series of injuries that resulted in a permanent loss of skating speed. Once that happened, he simply wasn't fast enough to keep up in the newly post-lockout NHL. It's really as simple as that.
As for the Thornton notes: Yes, he wouldn't have won the Richard without Thornton feeding him, that's not really arguable. However, he was a very good goal scorer before Thornton got here too. Like others noted, he got 28 on the 2003-2004 Sharks team, good enough for 23rd in the league in a year when the most scored by anyone in the NHL was 41. So Thornton got him to the Richard, but he was a top goal scoring winger all on his own.
Ultimately, it was just incredibly sad to see, because he was obviously trying as hard as he possibly could, and you could tell he wanted so badly to succeed, but his body just let him down. He is still beloved by the Sharks' fan base, pretty much all of whom still love the guy. I know I hope they offer him a front office job when he decides to finally stop playing.
