This is a personal decision, not one he's trying to push on others. Just as those who so choose have the option of living as long as their body holds out, so too should people have the option of not prolonging their life as long as possible.
I have inoperable cancer, and its effects on me are such that at some point I will no longer be able to manage the symptoms to the point that life will not be worth living. I don't want to spend months or years in a narcotic fog to dull the pain enough so I can just keep breathing. Short of spontaneous remission, I will at some point choose to end my own life rather than suffer needlessly. My family is aware of this decision, and I will inform them at the point it is necessary so there are no surprises. I would do the same were my mind degrading to the point it would be clear my existence contributed nothing more than consuming oxygen and taking up space. I've talked to many others who feel exactly the same way. I'm not trying to kill other people based on some arbitrary criteria, but I sure as hell won't accept being kept alive because "all life is sacred." It's not.