Its the difference between film as art and film as a commercial venture. FFC only made the film because he was going broke, and the studio wanted to exploit the franchise.
What cannot be criticized enough is the horrible performance of Eli Wallach.

IMHO a GFIII should depict the breakup of the Corleone family, the destruction of Michael's financial empire (legitimate or otherwise) and possibly the indictment, trial, imprisonment and subequent death of Michael Corleone. As TB
often says the moral of the story is "crime doesn't pay." I cannot think of a better way to drive that point home.