I think one can see from Michelangelo's sketches, and the process of various monuments like the Medici tombs, and the strange emergence of the last works more or less like a cloud forming, that Michelangelo did draft as he went. The Platonic "it was there all the time" was, I think, more for popular consumption. Like most sculptors of my experience, he was a problem solver -- stone has its quirks too.

I don't think, personally, that all art works are failures that didn't match the original impulse. Often they are better!!!

yours,

Peter T.