I see your point. I was watching Discovery of some such channel and they showed Stephan Hawking as he is today and someone in his condition I can see would have a hard time implementing an end-of-life act on their own. But even he could drive his wheelchair off of a cliff if he wanted to. About the only one (fictional character) I could think of who could not implement his own demise was the one wounded solider in the story about Johnny Got His Gun (supposedly based on a true story). That is one condition I can see not wanting to live through, although that character did.
That's something of a stretch don't you think? Given that he requires 24 hour care, I think Hawking would find it difficult to find himself at the precipice of a cliff, able to wheel off without at least one other person complicit in the attempt.