Do you think it so terribly unlikely that the entire construct was developed by people who simply did not want to face the natural consequences of being too old to hunt or gather any more? The fate of the old, as well as the injured, in those days would have been death slowly by starvation or quickly by being killed and eaten. Subsistence was a reality then, so those who could not contribute to feeding their group and kids any more would either be kicked out or utilised. Making up a tall tales or two about the spirits and how only the old folk could communicate with them would be as good a way as any to get a few extra meals, while you figured out what the spirits wanted.. Surprise, surprise, the psritis wanted the tribe to look after the old folk and do what the old folk told them to do.

It seems to have been an effective con, since we're still doing it!