He was getting old, plain and simple. I have noticed that some older people (75+ crowd) start saying things and taking positions contrary to what they had always supported. Some become downright kooky as a prelude to dementia. Unfortunately, I am going through this with my Dad right now who is showing signs of Alzheimers. He was always a staunch Republican, but the last election he voted Democratic 'cause he thought Bush would eliminate his Social Security.
"He was getting old, plain and simple."
I'm really glad you said this, because it is a fact of life--aging can slow down cognitive activity including the application of critical judgment and the desire to engage around contentious issues.
I recall a time when I was in graduate school; an elderly retired luminary of the field, known to our professor, happened to be in town, and graciously agreed to come speak with us for two classes.
A classic Freudian, he found himself being vigorously challenged by some enthusiastic grad students on some basic propositions. The next day he no-showed, and it turned out he just went home. He just didn't want to fight about it, so he didn't.
Maybe Goldwater just didn't want to fight about it anymore.