I have had a bug since Friday. In bed most of the time since. Completely off my food. Yesterday I think the sum total of what I ate was one Weetabix and two bits of toast. I have lost a few pounds, but since I am overweight it doesn't matter. A little of what you fancy is the best idea. If an elderly person has poor digestion, their worst nightmare could be a "good"meal, especially with someone watching them. In bed or a chair all day means someone is not going to burn many calories.
I think when people get very old, with or without dementia, and are inevitably within a few years at most of their deaths, then quite frankly they should be able to eat whatever they like!
If you are not active, then yes, you need very little 'fuel', and why not take that in the form that most elderly people like best, which is sweet carbs! Puds, biscuits, cakes, toast, whatever.
Why not?
I don't see that stuffing them with protein and vitamins and fresh veg/fruit - if they don't like it! - is being kind or helpful. If they've got a strong enough constitution to live into extreme old age (say 85 plus), then they won't rely on a balanced diet anyway.
The nurse at my MIL's (92 with dementia) care home was telling me that getting her to eat was difficult, but when I take her out she packs away an entire cream tea (scone laden with jam and cream! - sometimes one and a half scones!), so is clearly keen to eat that kind of food. The trouble is, at care homes I suppose they are under orders to ensure some kind of balanced died, with lots of protein/fresh veg, which I suspect most of the residents don't actually want or enjoy.
My MIL never 'ate greens' (willingly, or at all!), and for her generation greens were usually boiled to death anyway. She's always, like my own dad, preferred puddings and cakes. Meat was often disliked by their generation as again it was usually overcooked (eg, boiled beef, and rock hard lamb chops etc etc).
Obviously, if they enjoy a healthy died, fine, but I don't think they should be plagued or pestered to eat one, if all they really want are sweet carbs.
Just my take!