They can manage to stay by themselves but incase they got sick or got some dreaful disease like Alzhimers or Dimentia or even something physical then they can't do that. In that case we would have no option but to put them away in a nursing home where they can be cared for.
I personally think there is nothing wrong with keeping your parents in a nursing home.. They will be with people of their own age and background, they can make friends and at the same time be safe and well looked after.