No, it really wasn't necessary, except to appease the folks who wanted to know what happened to Michael. Now that we know, and have judged it isn't quite the way we would've liked, that's the way it goes.
I found out from the Mario Puzo "Biography" on A&E, that he & FFC actually disagreed on the GF3 ending; but Puzo felt that the "film" belonged to FFC, and therefore deferred to Coppola's vision.
So again, Michael's dead, Vito's dead, Sonny, Clemenza, Roth...they're all dead. The Corleone saga is over. Unless someone really, REALLY cares how Vincent runs the family, in which case they might want to go & write their own novel and hope it holds a candle to GF, that should be the end of it.

By the way....
Don Vito never said, "...I could've been a contender...", etc. etc. Those lines are from another classic Brando film, On The Waterfront.

Apples