I don't know about anybody else, but I was once asked to make the decision.
An aunt of mine, late 80s with advanced dementia (she had no children) was in a very nice care home. She had recurrent UTIs - she had always been prone to such things - and each time it would take a long time for them to get her back to some sort of 'normal'.
One day I had a call to say she was refusing food and drink after the umpteenth UTI. I was asked whether we wanted her taken to hospital and put on a drip, or given palliative care where she was - without the drip she was certainly going to die. It was an awful decision - all the rest of the family were away or otherwise out of contact. I was put in touch with her GP, who was lovely. I could only ask what he would do if it were his much-loved aunt.
He said he'd leave her where she was, where they'd keep her comfortable, rather than send her to a noisy hospital to be poked about by strangers - especially given that at her stage of dementia she wouldn't have a clue what was going on, or why. He said that given her history it was only going to happen again in a few weeks or months.
It took a lot of heart searching, but I did think this was what she'd have wanted for herself, if she'd been able to understand. She hadn't been getting any enjoyment out of life for quite a while.
A cousin who was back in contact, or I, sat with her for much of the time. She was nearly always asleep, and did not seem to be in any discomfort. Staff continued to offer fluids and things like yoghurt, but she would invariably close her mouth and turn her head away.
The CH staff were lovely, and everything possible was done to keep her comfortable. She slipped away quite peacefully after about a week.
Although it was such an awful decision at the time, I never regretted it afterwards. I'm quite sure the person she was before, would have said, 'For heaven's sake, just let me go.'
I would have made the same decision for my mother, if the situation had ever arisen, but thankfully it never did. And given that she was that much older than my aunt, with even more advanced dementia, I'm not sure anyone would have thought it at all kind to try to keep her going just because it was possible to do so.