1) Identify all mistakes made by the previous management and come up with a plan to correct them.

2) Get buy-in for your plan and execute it.

It might appear that 1) is harder than 2). But it turns out that the company's interests are not always aligned with each and every individual in the company, and implementing change always results in winners and losers. Getting the buy in from losers can understandably be quite hard. And then you also have the winners who sympathize with the losers and don't realize that they will also lose their jobs if the plan is not implemented.

When you're running a startup, a CEO can single-handedly turn things around. In a larger corporation, a CEO is going to rely on a chain of command to get things done. When you factor in the misaligned incentives or loyalty for people who are at risk of losing their jobs, it's easy to see how information channels can stop working and neither 1) nor 2) can be accomplished successfully.

I agree that brining in a 3rd party like McKinsey feels like a lazy shortcut to addressing the fundamental organizational challenges, but the more I see, the more I am starting to understand the upside of that option.