I always thought that he changed after Lindros tuned him. He was obviously a great player regardless but I wonder how he’d be remembered if he kept his physicality more, and also shot more like his early days .
The Lindros fight was definitely one reason. I think the other was self-preservation. Thornton, as a young power forward, was already struggling with injuries. He missed more games between 2001 and 2004 (ages 21 to 24) than he did between 2006 and 2017 (ages 26 to 37). And he was heavily criticized by Boston's media and fans in 2004 (he had broken ribs, but played in the playoffs anyway, and was held scoreless in seven games - he was damned if he played, and damned if he didn't).
I wonder if it was Thornton himself who decided to change, or if he was steered that way by his agent and/or coaches. Many of the top power forwards from that era had all kinds of injury problems, so in hindsight, Thornton's change was for the better. (Plus, of course, there was less emphasis on size/strength after the lockout). Still, he would have been an amazing player if he continued to play like he did in 2001 (but it's unlikely he would have come close to 1,700 games).