Moving the deckchairs in the boardroom isn't going to solve systemic problems in a company.
Well, that really means tearing the company up completely. I can't see how a new board could effect reform. And replacing all the middle management would destroy the company.
And as you say, it takes time for a new culture to move through the layers; but the PO doesn't have time. Does Boeing? I realize the companies are very different, but both have marinated in an unhealthy culture for a long time.
Because that's how the CEO makes big change happen: management -- from the C-suite down to line managers -- needs to get on board with that change. If they refuse, they get fired and are replaced by people who will carry out the change. If this process will take too long to save the company, then... again, maybe the company needs to fail.
Take a simpler thing: if a new CEO sets an example of taking a decent amount of time off and promoting that as what people should be doing, because they value mental health, then that will gradually trickle down into upper and middle management, and down to the rank-and-file.
Management types who resist that will be replaced or marginalized, because this aspect of culture matters to the CEO and this is something they want to promote. It won't happen overnight, but ultimately management will be filled with people who are taking solid amounts of vacation and push their reports to do the same.
A CEO that wants to push an engineering-first/safety-first culture will be firing executives and management types that try to hide problems or push through things that don't meet the quality bar that the CEO is looking for. This sort of shift will not happen overnight. It can take years. Whether or not a company like Boeing can survive this sort of change, and if they even have years left to make that change, is another question, of course.
But I argue the opposite: the CEO (critically, with the unwavering support of the board, which requires the support of shareholders) is the only person who can make this sort of shift.