The Rolling Stones after Brian Jones. Whatever Jagger is, he is not Rock and Roll. Their output has been dire for decades.
Prince. Apart from a mild Hendrix influence, his whole career has been based on scandal and hype. Ultimately only the songs remain, and I can't find a single one I like. Even Raspberry Beret runs out of ideas too soon.
Can't think of a third act, but have to admit that Adelle and Mariah Carey have the ability to induce pain whenever they open their mouths.