What plans does anyone have for when the quality of life is so diminished that there's no point in going on?

Prayer!!

When I was declining rapidly I wondered often about when it would become intolerable. I was impressed by people such as Stephen Hawking able to continue leading an apparently satisfying life despite such grave physical disability. I didn't believe I could do similar.

Fortunately I no longer think I'll have to find out if I can handle becoming severely disabled. The fear of it was great motivation to maximize my health and fitness and despite having sunk to a very low point I've been able to regain sufficient fitness to both want and believe I will live a very long and most excellent life.