May you never find yourself in that position.
The point is, we don't want anybody to find himself in that position.
If euthanasia is permitted, it opens the door for fraud and surreptitious murder-for-gain. I guarantee you that there are many wicked but cowardly people in the world who would take advantage of that door once it's open, while they aren't brave enough to take a lockpick or a sledgehammer to a locked door.