I always thought it was just a film and not too get bogged down in the detail, but one thing that bothered me is the ending.

Michael has the heads of the 5 families killed and thereby "settles all family business"

Why should those hit gangs be subservient after that ? Were the Coleones (excuse spelling) subservient after Sonnys death (The affected leader) ? No way. Is not one of those gangs going to attempt some kind of revenge ? Of course they are, what kind of gangsters are they going to be otherwise.

I like the ending of the UK film The Long Good Friday when the gang leader (PLayed by Bob Hoskins) has the main players of the other side(the IRA) killed at a meeting at a racetrack. Then he thinks hes settled it, problem solved. Of course, at the end of the film when hes relaxed after his victory, he gets in a taxi. But the taxi speeds away, the drivers mate turns round and points a gub at him. Its the IRA, and hes had it. (The film ends there)