I don't know the reasons why Banks stopped playing SS, but I can imagine that he was getting older and it was something he couldn't handle long-term, or he was beginning to cost his team defensively. Or the Cubs wanted to preserve his bat long-term and didn't want him wearing down. Not far off from the reasons ARod isn't playing SS. And again, I don't know why this matters.
Players just aren't that good in their 40s. If ARod puts up .250/15/80 is that acceptable to the Yankees? If there's any team who can eat the money and not play ARod full-time, it's the Yankees.