I read an article a little while back that posited that being able to endure boredom was one of the biggest challenges we face in our work lives and that being able to endure boredom at work was becoming a key to career success. (Sorry don't remember where I saw it.)
I know this has played out in my own life over and over. Once I have grown creatively/professionally/technically to a point where I've mastered each the various crafts/skills/task in my life/work (most being non-photographic) my interest in them tends to wane. I need creative challenge/growth to maintain my interest. I've given up on both failing and successful career paths to improve my life.
Specifically related to HCB's choice to switch, I fully empathize with him. I have experimented a bit with painting and drawing and it has some very distinct creative advantages over photography, like contrast and exposure controls, there are simply no burnt highlights or blocked shadows unless I paint them that way. Being able to choose the color and brightness at will between canvass white and the black of the paint allows adjustment of the subject matter at will regardless of reality, the mood/feel of a piece can be changed from right to left at my whim, the laws of physics don't apply to the subject matter/ideas I might choose to paint, the list goes on but you get the idea.