I am definitely terrified of this and it's a huge reason for me wanting to CTB. Aside from the possibility of being old and living in a nursing home, there are so many other horrible things that could happen even while I am still young. I could get in an accident with a drunk driver on my way home from work one day and end up with severe physical disabilities that make it difficult or impossible to take care of myself if I get injured badly enough. I could end up getting cancer and having to go through a bunch of surgeries or chemo and being too sick to do anything (unless I am diagnosed early and can CTB before it gets bad enough that other people have to make these decisions for me). The worst outcome of all would be losing the ability to end my own life and having other people force me to stay alive because they can't let me go, regardless of how it happens.
It would be better for me to die young while I'm still somewhat healthy, before any of those terrible things can happen. The future with all of it's dark, hidden possibilities terrifies me so much that I have no desire to see it. Sometimes I wish I could be like everyone else and pretend that nothing bad is going to happen to me, but I can't. I can't live a happy life knowing that these things could happen at any time.