My grandad had a stroke 20 years ago, and since then he has had what has been called dementia, but it was more like side effects of the stroke. Like, he knew what he wanted to say, but couldn't articulate it, and this made him very frustrated. At the time of the stroke he was still physically able to be out and about - accompanied but unaided - which made a huge difference, but as he got older, this changed, and he was basically housebound with carers, unable to dress or feed himself.
He developed an infection a short time before he died, and was hospitalised for it. My grandma was adamant that he was treated for it, because she was terrified of losing him.
She is 20 years younger than him, and the past two decades of her life had been spent being his carer rather than his partner .. but I think she was scared of losing him because her life would be so empty, and even though he was the way he was, there were 'VERY' rare times where there would be a short flash of normality, and he would remember something or say something that actually made sense (possibly just a fluke).
He wasn't eating in hospital, and wouldn't accept being fed, and they discussed what the kindest thing would be, and my grandma insisted he was tube fed as that was the only chance he had.
My grandad used his last remaining strength to fight off the tube, and said 'no. enough now' in the most decisive tone he'd used in 20 years.
They agreed it wasn't in his best interests to push for this, and he died the next morning at the age of 96.
What I'm saying is that it might be so, so obvious that someone has no quality of life left, but there is still the memory of who they were, and some loved ones want to cling onto this, and do so by doing everything in their power to keep the body alive - even if it is only a shell of the person they knew and loved.
We all agree that my grandad needed to go - particularly given the way things are this year, we're so, so glad he didn't survive to 2020 as he wouldn't have been allowed visitors, which would have been heartbreaking.