An important point! Many people are terrified of disability, and think they'd rather be dead, in theory. They often find they'd rather be alive, when faced with the situation in reality.
It's not, I believe, that the fear of death is so overwhelming that any life is preferable, but rather that the fear of disability has been tremendously exaggerated.
Our culture holds that a "meaningful life" is one that is productive, and that "dignity" means autonomy. The disabled and those who work with them remind us that these ideas are false.
(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)
Actually what it is, is that we have a God given ability to adjust to most disabilities. I think that you are right that most people are so self-centered and vain that the very thought of not being like everyone else makes them think that they had rather be dead, but then when something bad happens they change their minds. Better be careful in what you say that you would do in any given circumstance until you are there.
The "meaningful life" argument is a dangerous and utterly corrupt reasoning that is often used by the death lovers. The flaw in it is easily exposed thusly: deciding that a life is "non-meaningful" and "not worth living" is a lot easier when that life is not your own.