Their downfall was pride: they were so proud of their intelligence and competence that senility and incompetence seemed to them worse than death (I wonder if they would have supported killing the young mentally deficient as well, but that's a different subject).

But I've had relatives who were senile and incompetent, and it's not actually bad, when one is surrounded by family and friends. We were all born dumb and helpless, and many of us die that way. A senile, frail old man is no more worthless than is an infant. Sure, he won't ever grow out of his state (as an infant will), but he once was otherwise and has hopefully earned some love and respect.

There are worse things than helplessness. I think hubris is one of them.