>>Without the retrospective, it's just modified waterfall.

This is true. But retrospectives will never work in any reasonably sized team, the reason is middle managers don't like being told or like being talked about their work in front of everybody, or being told to change something. Ego plays a big role in these things and they don't like being given feedback or advice, especially because these managers tend to think themselves as people in authority as all knowing and in complete control.

This is human psychology, developed over thousands of years. Masters never took feedback from slaves, so why should the modern incarnation of the older masters take advice from the current day slaves?