It's a personal choice, and in no way would I advocate it for everyone. But I, personally, don't want to go from being a productive member of society to a drain on the emotional and financial resources of the ones I love.
I didn't come from a wealthy family, and I had to watch two of my grandparents spend the last decade or so of their lives dying of various conditions. My grandfather had dementia, and got a pretty raging infection from diabetes he didn't know he had until he was in his 70's. My grandmother worked until she couldn't work any longer, and died from cancer six years later.
My family did their best to take care of them both, put them in as nice a place as they could afford, and we went to see them at least once a week. It was tough on everyone, for a plethora of reasons. It was hard for my grandparents, who wanted to continue participating in family events, and wanted to feel useful. And yet they couldn't, and it truly hurt them.
So, I don't want that for myself, or my family. It doesn't matter how rich you are - when the mind and the body are beyond the point at which you can maintain some level of happiness, I don't see the point in living.
As I tell the folks at the poker table, "You play your hand, I'll play mine." If you want to live to be feeble and have no bowel or bladder control, and be a drain to your family in some way, by all means you go for it. Me? I'd rather end it.