Thank you for sharing your thoughts. For many people dementia is the most feared way to go. One of the big challenges is that this is not an all or nothing condition, it is a gradual decline, making it hard to determine when one's old self is gone.
Hawaii, California, Oregon, Washingtion and half a dozen other US states have Death with Dignity laws in place, most of which require a doctor to certify that you have less than six months left to live. Compassion & Care has a wealth of information about end of care decisions and rights.