I shudder to compare Duvall to that *&(*%$(*&* T.O.
Duvall is a great actor, and by the time of GF III he was into financing some really great indie type movies including the one where he plays a fallen preacher, and also Sling Blade, for which he put up his own money. I think there were other projects too. I think he knew very well that GFIII would not be what the first two were, and that it was being made as more of a money making deal than an artistic one. He was right to hold out, and if they wanted him they should have paid him.
I dont know if the story would have been better with him or not. I mean he is certainly ten times the actor George Hamilton is, but on the other hand I think Tom Hagen was a better lawyer, and he would have seen through that "Subject to ratification in Rome" loophole that Harrison seems to have overlooked.
Also I guess they would have had to explain the whole mistress thing with his son having become a priest and all.
Finally, if Tom were around, I don't know if Connie and Vincent would have been in a position to order the hit on Zasa.