Really? Are you forgetting the evolutionary advantage that older women could provide, in primitive societies, to their children and grandchildren? From caring for the young, retaining and passing on knowledge, to food preparation while the adults are out hunting and gathering -- the list of beneficial contributions is quite long.
And with evolution, all of this counts! If you help your grandchildren survive to reproduce, your genes are passed on to still another generation.
Things do not just develop because they are good in evolution, they survive because they aren’t sufficiently bad.
It is most likely that certain traits come into being for no particular reason and continue if they are good or not sufficiently bad.
Take a sabre tooth cat for instance. Assuming that their canines did not suddenly grow six inches, but grew gradually, there is absolutely no competitive advantage to having teeth that are 1/2 mm longer. Therefore, it is far more likely that the a genetic code change created ever growing teeth. That the cat found advantage to this was adaptation, and may have reinforced the evolution. When the teeth became too long, the trait was sufficiently negative enough to cause a change in the gene pool.
Taking post-reproductive women as the specific example. It is the height of creative silliness to think that women evolved to be unreproductive caretakers. Any creative person could easily generate a half-dozen alternative theories explaining why it developed. In fact, it is simply wishful thinking to crudely elevate any positive feature to the role of prime selector.
Why do human females cease to reproduce in the middle of their lives? We don’t know. All we know is that they do, and that the result is not so negative as to eliminate the species.
Your logic applied to the average workplace would make lazy, unproductive workers essential, based on the undeniable fact that they are there. Thus, whatever they do provide must be a positive. In truth, as long as they aren't sufficiently bad, they will likely remain. Although evolution might provide a better worker, it will not eliminate the old one unless that worker is sufficiently bad.