I've read the argument "III was unnecessary" before, but I have to admit that I don't totally understand that philosophy. I kinda do and don't.
I get that Michael lost sight of what was more important, but I don't completely see why it makes a third film unnecessary.
Well, and this is just an opinion, but 1 & 2 are a complete unit unto themselves. The tale of a family, that roots its story in the juxtaposed philosophies of it's two patriarchs. The former being a tale of the failure of Vito, who intended his eldest to succeed him, and had never intended his youngest to do so, and of how it was destined that Michael come to power the way he did, not because he was the smartest, but because he was the most cold & calculated. Fredo was naive & Sonny was ill-tempered, and it is that attribute of Michael's which shapes the future of the family. Pt. 1 is the reign of Vito & the rise of Michael
Pt. 2 is the B side, the reign of Michael & the rise of Vito, told in flashback. It is the tale of how something as powerful as a mafia crime empire can be built from the simple beginnings of a man intending to make a good life for his family, and how that same powerful empire can be brought to an end by the failure to recognize the importance of that family
The story of the Corleone empire and its two patriarchs is two sides of a coin, night & day. A coin doesn't have 3 sides. The 3rd film is a faint echo of the 1st, with different set dressing. Half sequel, half reboot, or whatever they call that, when they carry on the same story with new faces. It's generally considered a weaker film than the others. Well, why is that?
The acting from Pacino & Garcia is wonderful. The cinematic production values from the director are just as good. As a stand-alone film, pt. 3 is not all that bad. As a chapter in the Corleone saga, it is a coda at best, and ultimately a let down, because the whole point of it was established already at the end of the previous film anyhow. By the end of pt. 2, the history is in the book. The writing is on the wall, as they say.
To push that story onward, the way they did is really just a gratuitous money grab. They might as well have done a 4th , at that point, to at least give closure to this new ill-conceived element they introduced. That's why people want a 4th. The story was done. They added something they didn't need to, and it left people felling like it was open ended. Pt. 3 is a new coin that people want the flip side of, but I think the whole addition is unneeded. It's not a serial, especially if it's only to tell how a new Don comes to power. It's a contained story about how two patriarchs are the lords of their shared family at different times. Godly fathers.
That's my take on it. I suppose the notion of a "3rd film" in and of itself isn't categorically unnecessary, but the addition of
this 3rd film is, and I personally can't think of a 3rd that would be necessary