Even the best idea will face the hostility of a management that brought the company to where it is today. It never works.
Everyone who is there now and especially rose through the ranks at some point stood behind and toed the line that brought them to that point ("Cheered for more desktop ads, agreed and liked the 'Mavens' metric"). Now they'd face a significant cognitive dissonance if they did an about face and said "Yap that was stupid, what were we thinking!?". It just doesn't happen.
It can only happen with new people, so a lot of existing ones have to be pushed out of the door. But new ones take a while to on-board, they are outsiders and so on, they'll face internal resistance in the changes they propose.
It's the downward spiral of doom. They'll start laying off people to save money. The more people they lay off, the less likely good people would want to go work there. The less progress they'll make, so they'll have to lay off more people etc.
McKinsey isn't going to get them there. That's a move of a dying company. Marissa Mayer needs to do this herself. If she doesn't know who the right people are to keep versus release, that's a bad sign.