People should be able to choose for themselves... Both my parents suffered and I know for a fact that my father would have taken the option to end it all earlier because he knew what was coming. There comes a point in your life where the quality of life is no longer there anymore and it just becomes a struggle to get through each day and that is no way to live.. It just becomes tedious survival with no happiness...
I am sure that there would be other family members that might oppose it but that is probably down to some selfish reasons, nobody likes to let a loved one go but let them choose and give them some dignity or you might end up with the situation @Devil-Dick Willie mentions where the choice eventually passes you by and you are trapped in a body that hardly functions and I don't personally fancy going out like that..