I'll grant you that popularity and quality are not equivalent. And there may be some obscure giants that many of us don't know. But in my judgment "greatness" encompasses both quality and renown; if you're not well known, maybe your greatness is limited to the small circle of folks who care rather than the general public, i.e., posters on a message board.

I agree there are many ways to make music and singing, for example, is one of them. I understand "musician" is defined both broadly and narrowly. In common parlance, I think most think of a musician as one who plays an instrument and an artist as one who musically performs. Thus, George Strait is one heck of a singer, one of the best artists in his genre's history, but as a musician he doesn't compare well with folks like Vince Gill, Brad Paisley or Keith Urban.

But I'll defer to people who take the subject more seriously than I do.