I know it is a very sticky legal issue, but on the other hand, it seems that it is something intensely personal.
It might be personal to you, but to your Health Insurance
company it's all about money.
Can you imagine the "big discount" you could get on
health insurance if you signed that form giving them
permission to "relieve your loved one from excessive suffering"

People will sign this stuff ahead of time, not really knowing
the implications. Then one day when you take granny to
the hospital for some minor ache, they'll decide that
it's too expensive to treat her and they'll
"relieve her of her suffering"

It's a bad idea.