Hate to say it, been faced with it myself (forgive me for having personal experience), but if they are that far gone, do they even know that they are voting? If not, who is casting that vote for them? Bedridden, in a nursing home?
My MIL is in a nursing home,; she decided her experience would be one of helping others, she's been in 10 years, but she's still alert though she can't straighten her back as well as she used to. I have no idea which way she votes, but since her mind and person are still engaged, she should continue to vote.
You're right, though. They know when they no longer wish to vote, or if they get too bad off, they will usually indicate their preference. At that time, it's only fair to let the voting lapse, not substitute somebody else's opinionary into who she should or should not vote for in their better interest or kickback from a corrupt voting precinct person.
I'm really disappointed that fraud is so rampant these days. There are so many lawsuits, it tells me people's word is not always their bond anymore, and that's dismal.