If there is anything that scares me it is dementia. My grandmother suffered from a mild case of this and it really affected me. I seriously hope to side-step this later in life, either by luck or by choice.
I've witnessed the flip side of this: a grandparent with a hard life plagued by worry amplified by the confusion of Alzheimer's. Slowly wasting away for a few years. She did love her sons and grandkids playing piano up till the end, though.
It is truly a beastly disease.
Some things are worse than death.
This can get pretty hairy from a legal point of view even in places where it is permitted when the patients faculties are diminished.