She may have a learning disability and need care, but she should be treated as a whole human being with the same rights as anyone else.
But she needs to be treated as the whole human being that she is - there is no default 'whole human being' that applies to everybody.
As has been said, for most of us in her position, it would be a case of getting our affairs in order and making sure everything was known/passed on to our loved ones to take over. We'd know we couldn't stop the dying, but we'd focus on what we could take care of - indeed probably with a lot of worry. Her affairs are already looked after by others, so she has nothing to worry about there.
Rights come with responsibilities and, sadly, her inability to take on some of the latter already means that she also doesn't have equal of the former. To compare her case to children again, they don't have the right to decide on which house to buy/rent - how big, location, facilities, price etc. as this right is ultimately down to adults; but equally, when it comes to the concern of having to find the money to pay the mortgage/rent each month, and to meet all of the bills and put food on the table, the children also don't have any responsibility for that.
In fact, continuing with the likening of her circumstances to those of a child.... we shield children from films/documentaries telling stories of the harrowing deaths of other real people, because we know it would frighten and trouble them; so why would we dwell on telling them of their own deaths?
I'm with you: I would hate not to know if it were me, and I can see why you're wanting to defend her; but you and I are fortunate enough not to have learning disabilities and would be able to deal with the whole picture in a 'standard' adult way.