Have you fleshed out what that might possibly mean, or how that could ever happen? Unless our body/mind is "naturally" declining (i.e., because of nature), there would seem to be no reason for a well-functioning person to choose to die. Nature is always going to dictate the most important terms.
-Plain old ennui. Don't underestimate this.
-Depression from alienation because society and culture has evolved over centuries beyond their capacity to accept change or has evolved to a point that society no longer accepts them.
-Grief at the death of loved ones, either accidentally or through their own choice to check out.
-Grief at the estrangement of loved ones as their personality and beliefs change over time.
-Loss of one's accumulated wealth and privilege, whether by accident, incompetence, or malice of others
I'm sure that many more are possible.
I would add that basically everything you list is why there exists in human cultures a general disapproval of suicide, because people may feel bad in the situations you list, but suicide is not a good answer; in most cases the person's feelings will change and they will be glad they chose not to kill themselves. Deciding to die for reasons like those listed is not generally considered "dying with dignity"; it's a tragic case of people becoming overwhelmed by feeling to their own detriment.