I imagine a whole lot of people could do a better job than Yahoo's new CEO, for example. At that level, being the CEO of a corporation with that level of money that already runs itself, you could likely get by doing pretty much nothing/just going along with your advisors'/the board's advice to keep the company at least in the black.
It's often hard for people to appreciate the scale at which the CEO has an impact. As an individual engineer, if you're replaced by someone who's not an engineer, they are going to fail big time... But the amount of damage they can go to the company is very limited. Someone else will just pick up their slack, and the financial loss to the company is likely less than a million dollars.
At the CEO level though, even if you don't fill completely, even if you're 99% as good as the previous CEO, that 1% difference on a massive company translates to a loss of tens/hundreds of millions of dollars. And that's assuming you're 99% as good. If you're only 90% as good, the damage is even greater.
Can you scrape by as CEO without bankrupting the company? Sure. But there's a reason why boards are still willing to pay millions of dollars to hire the best possible candidate. Because when you're managing assets worth billions of dollars, scrimping on a few million is simply premature optimization.
