You can't keep them bed bound for N years until they die. The problem with severe dementia is that people forget - not only where they live, but whole swathes

You can't keep them bed bound for N years until they die. The problem with severe dementia is that people forget - not only where they live, but whole swathes of time. They forget people, places, faces, all sorts of things (varying based on their particular condition). So they wander off, trying to go home or just to go somewhere else. In many nursing homes this is handled by having locked wards, which are often completely indoors or have small fenced areas. This is a more village like aspect, so someone can go and do normal activities without putting themselves at risk.