I agree it was the point. And, even if another actress had played the part, I still might not have liked the portrayal of Kay. To be fair to Keaton, Coppola must have directed her to play the role the way she did. Kay came across as bland, whiny, weak, and gullible. I found a site that claims Mia Farrow, Blythe Danner, and Michelle Phillips also were considered for the part, and I'd imagine they would have portrayed Kay pretty close to the way Keaton did, as Coppola must have wanted.
No offense to Keaton. I'm just expressing an opinion here. Her tone of voice and mannerisms are more suitable in other movies.
It's fun to think of other actors and actresses in these roles. At the moment, I cannot think of another actress in the 1970's who might have fit the role of Kay, but I can think of a few actresses who fit the picture of an upper-crust WASP. Sigourney Weaver is one example, although she would not have been suitable for the part of the vulnerable Kay, either. But, an actress like Robin Wright or Suzy Amis might have worked. They were still children when The Godfather was filmed, but an actress with their tone of voice and temperament would have served as a stark contrast to Pacino's portrayal of Michael.
As a side note, many people on this forum talk about "white culture," and they want the word "white" to be capitalized. The Godfather illustrates there is no such thing. There are big differences among European cultures.
Speaking of the wrong actor in a part, I've never gotten over Pacino in Scent of a Woman—a role that absolutely, positively should have gone to Robert Duvall, I don't care how long they would have to wait for him. Duvall is even married to a South American tango instructor. Al just didn't carry off the military officer thing, IMO.
And I totally agree with you about “white” culture. There are marked differences from one of the original tribes to another -- the Celts, the Normans, the Etruscans, the Romans, the Scots, the Angles, the Saxons, the Slavs, the Teutons, the Norse, etc. Even today, the Catalonians are trying to break away from the rest of Spain.
However, the same is true of African-descended folks. Their tribal differences and ethnicities run a wide gamut, but they are all lumped under one label, “black.” Just goes to show that skin color is not the issue. Culture is.