What If Michael Had Said...
...."Sorry, Guys. I love my father and will do whatever I can to help him recover, but you are planning a murder, and I want no part of it. You guys are the in the family business, not me. Do what you want, but I'm out".
Seems to me that Sonny eventually becomes Don and things fall apart from there, don't you think? I know it's a hypothetical and the film doesn't go in that direction, but still, it's an interesting thought.