trismes,
I'm sorry if I sound harsh, but to me this reads as if you are saying that if other carers made more of an effort, they could get the person they are caring for to eat a healthy diet, and that the person with dementia would understand if the "alternative future" were spelt out to them.
Dementia is a very individual illness. My husband, for example, had been in the "natural health" field for many years and had qualifications in herbalism, naturopathy and other complementary therapies. For several years, he travelled, lecturing in diet and health, and the use of herbs and natural remedies. He was a very firm believer in the "Mediterranean Diet", healthy oils, etc. etc. And in the very early stages of his illness, he did indeed do a lot of research and stick to an excellent diet and recommended supplements.
However, as his dementia progressed (and progress it did, relentlessly and in a particularly horrific way) his tastes changed totally. He refused to eat the foods he used to love. The olives, hummus, olive oil, and all the vegetables & salads. All he wanted was, to be honest, ice cream and bananas! And occasionally, sandwiches. His beloved sweet potatoes were suddenly spat out. I found food hidden all over the place! Under tables, under shelves, in bookshelves!! Getting him to drink was a constant battle. I have him a fresh glass of different drinks at least every half hour. He would take a couple of small sips and then leave it. So, eventually, he did end up in a Nursing Home. Not because I or he "failed" in any way following some diet or getting him to eat properly. It was because he had dementia, and it eventually took an aggressive turn. He needed 24 hour care. And actually, as it turned out, he did much better in the nursing home, he even ate better.
Occasionally, someone will find an approach that will work in their case. But dementia is something that as yet, does not have an approach that works for everyone.