I too feel it is best to talk about the end well in advance, because when you are in the thick of it you are too wrapped up in practically/ physically caring for your loved one, dealing with your own emotions and trying to keep a 'normal' life for them to pick up on what they do not say.
once the end has come you have time after to think what if, but it really does not help to dwell on what ifs, better to put you energy into remembering the good times.