John,
An interesting philosophical point! One could say that we are not discussing reasons to continue but reasons to stop flying. If one learns that one is not as sharp as one used to be, that is a useful lesson learnt that may keep one safe and thus justify not quitting. On the other hand, it may be the impetus to "quit whilst you're ahead".
Second thoughts, I think that is what Whirlybird said, but she said it in English!
Rgds
L