Don't be too scared - the way we die now is still much better than the vast majority of humans that have ever died, even in the throes of dementia. The transience of health (and, by extension, life) is the great tragedy of all sentient life. The transience is a brute fact, the tragedy is an emotional response. We understand the brute fact very well, but can't change it. We understand the emotional response very little, and yet we have the ability to change it significantly.