Re: MS, true, it sucks. My friend's thinking was on the lines of:
1. Vanity - she could (and did) still look good.
2. Mobility - she would keep it as long as she could.
3. Optimism - if they could ever discover a cure for her nervous system, she would have less muscular atrophy to overcome.
Another example that leaps to mind is my former senior aikido sensei (then a godan, but I think he is now rokyudan) in Orlando. In his 70's, suffers from myasthenia gravis, yet as of a few years ago was still leading a large dojo and traveling the US to conduct seminars. Might still be, but I haven't seen him for a few years.
Again, it's easier to make excuses; it's even easier to convince oneself that something isn't worth the effort - defensive mechanism so the ego doesn't have to consider that it might be lazy.