I don't know. It works for me. This may seem like sacrilege to even say, but while I loved Part II, it hardly took the plot of the first film in a new direction (for the Michael stuff... obviously the young Vito sections were new, even though they were intended to parallel Michael... even though it was being transplanted from the original novel whereas the Michael parts were original). To clarify to avoid flaming, I love Part II. I love what they did with Fredo, I love what they did with the relationship of Michael and Kay. However, to me, it seems like an extension of the first movie which already established that a good person could turn evil. In Part II, we just see the evil person continuing to do evil stuff. He gets worse, and, in effect, his life falls down around him because of that, but, like you said, ChainsawAsh, he didn't change. There was no development. It was just Michael doing stuff. And again, it was stuff I found entertaining and fascinating, but it was just Michael continuing to be Michael.

Granted, it could have ended there, and it would have been a satisfying end. But I just found it so fascinating that Part III took Michael in a completely different direction. I don't think it's at all out of character that an older Michael might finally see the error of his ways and attempt to rectify his life and his legacy. People do that all the time. But I also loved the futility of it, the, "Just when I think I'm out, they pull me back in!" that no matter what he does, he can't escape his sins, and he still ends up just as alone as he did at the end of the second movie despite his genuinely good intentions.

And, yes, Sofia Coppola didn't help the movie too much, but she certainly wasn't a deal-breaker for me.