Certainly turning Yahoo around was never going to be easy. And it can be difficult to distinguish between "impossible" and merely "hard." We cannot possibly settle that question here and so there's no point in getting deeper into those weeds.
I will point out this though: I'm choosing to give Marissa and the Yahoo board the benefit of the doubt and assume that they disagreed with you, that they all thought it was possible. I'm also giving them the benefit of the doubt in assuming that they weren't necessarily wrong. Because the alternative is to believe either that 1) they are all idiots for thinking it was possible or 2) that they hired Marissa knowing full well that she was going to fail no matter what she did, and she accepted the offer and pocketed the $36M for her first six months on the job knowing full well that she was going to fail no matter what she did. That seems to me to be a much more damning accusation than anything I've said or implied. That they undertook a clearly impossible task is not actually a defense for a CEO who accepted an eight-figure salary IMHO.
What I do know is that at the time of her hire, on these very boards, people were singing her praises for the most part. Because the Valley protects its own.
Steve Jobs isn't the benchmark here. Maybe it was an impossible task to right the Yahoo ship, but what exactly did she do that was positive or innovative...at all?
I'm saying something simple. You claim that Mayer had 4.6B of cash flow to play with turn around Apple. My response is simply (a) no she didn't, and (b) to the extent she had real revenue to work with, it was an encumbrance: she was tasked with turning around a company for which 30% of its income was people routing their search queries to Yahoo. It's hard to think of big moves you can make that don't somehow jeopardize that revenue stream. She's boxed in.
(Then you look at the activist shareholder critiques of her tenure, and it's like, "she spent 10MM buying iPhones for everyone on her staff", and I'm like, it's fucking 2014, almost nobody but Apple has figured out how to make huge amounts of money on mobile but pretty much everyone thinks it's the most important thing in the world to do, and she's, what, buying the staff computers to do that with? Sheesh.)
Did she rise to the occasion and do something unprecedented in business history, like Jobs did? No she did not. Executive hired to do the impossible fails to do the impossible, film at 11.