Re the subject :- It's always happened , especially the Quitting angle. It's evolution, e.g. you can't make a reasonable living no matter how good you are, perhaps the shine of doing it for a living wears off and the mundane travel is also very difficult. There are probably quite a few other reasons. Groups are almost impossible to keep "on the road" because of cost.
Loads of top class performers have quit - but I believe most are happy with their decision and they rarely lose their ability, so they still retain their art. Problem is , they don't seem to get replaced.