A CEO absolutely can turn a failing business around? Are you sure of that? I don't feel any cognitive dissonance in my belief that, yes, Mayer is responsible for Yahoo's ongoing failure, but also it's quite likely impossible to turn Yahoo around.
Also: Yahoo! had a ton of resources in a rapidly-growing industry. I highly doubt turning Yahoo! around would be impossible. Hard, maybe. And maybe many people had already sort of given up the game and moved on to better things. But, again: If you're CEO, it's your responsibility to turn the company around. That's why you get the big bucks.
What I was commenting on was the implication that any company can be turned around. I don't think that's true.