An interesting question for sure but just to add a different dimension consider this.... among past generations of tribal peoples whose culture and spiritual beliefs are holistically intertwined with the physical and mental it was common practise for the old people to go off or be left to die a quick and painless death when they believed it was their time to go. They simply willed themselves to die.

In consideration of this I believe that as spiritual beings having a human experience we have the capacity within us to determine or perhaps more accurately align with the limitations or intentions of our human life cycle. If our 'eternalness' refers to our spirit and not our physical than it suggests to me that the physical is an aide or tool or temporary expression or manifestation of our spirit. It further suggests to me that spirit can command a physical response at will. Therefore should a person truly believe their time in the physical is over or no longer serves a purpose then they have the power within by spirit to cease to exist in the physical.

At the risk of really sounding like a wacho here I will suggest that I have personally experienced such power of spirit. I believe I brought on my own near death only to at the very last minute opt to change course and chose or be prompted to chose more physical life instead. I vividally remember a huge casp of breath pulling me back into consciousness after making the determination to will myself to die. Ever since that experience I have abandon any thoughts of medically assisted death. Should I believe my life is no longer worth living I believe I have the power without to will that to happen. If my desire is aligned with my spirit and is of spirit than I will cease to exist physically and move on in spirit.

On a more 'rationale' or rather 'worldly' note.... I also don't think it is fair or reasonable to put the decision of my death onto anyone else but me. My mother died from an over dose of prescriptions drugs. She made her own choice and used her own hand to go to sleep and not wake up again. That was her choice and I understood it even if the loss of her in my life still pains me almost 30 years later. To me the idea of assisted suicide or euthanasia would only be viable if a person deemed to be mentally capable to choice was physically incapable of doing it themselves.