That's easy to say when you're not the one in the painful position of watching a loved one deteriorate, imho.
It is painful, but you should rather rejoice in their life. I rather make their last moments worthwhile and try to cherish them, than think what could have been.
Obviously, if I'm ever in the situation, I'll find it extremely hard to follow my own advice, but it doesn't change the fact that it is true.
