Uh, well, Vincent is primarily FFC's creation, maybe there is more to be told with his story.

Michael died in 1997, we know from the GF DVD set bonus materials. So he lived 17 more years after Mary's murder, certainly something happened in that time period.

Plus, you have Sonny's two legitimate sons, Connie's two sons, and Anthony who might attempt to get involved with the business.

There is plenty of material which could serve as the basis of a GF IV storyline. It might suck but it might be good. It's pretty stupid to have a closed mind and just decide ahead of time to hate it.

If that reasoning had been in force, there never would have been a GF II, because after all, the Godfather was about Vito, and Vito was dead.