My friend John Barach pointed me to an interview with Peter Jackson in which Jackson explained the reasons for the change in the ending of the LOTR. Jackson did not, he claims, want to make Frodo heroic; he wanted to leave Frodo with the sense that he failed.
Jackson said, “we still tried to preserve what was important to Tolkien — the sense that it was the pity that [resolved the conflict.] There’s nothing that takes away from that. If Gollum hadn’t been there, if he had been killed earlier, then Frodo would have just kept it. We still had the presence of Gollum being the catalyst that led to its destruction . . . . We didn’t want to make Frodo heroic. We wanted to make Frodo feel that he had failed. At that point, he’s free of the burden — the Ring is destroyed and it’s no longer having that power over him. There’s a sense that Frodo feels like he wants to let go, he feels that he has failed, and Sam says, ‘No, don’t do that.’”