"The expectation to get a reliable estimation is unrealistic."

In ~35 years working as a software developer I never met an upper level manager who did not want and expect to get reliable estimates from developers. I never met an upper level manager who would accept the idea of starting any project without knowing when it would be completed and how much it would cost no matter how often those estimates turned out to be wrong.

Management is about control: of people, of deadlines, of costs, of outcomes. There is no way the majority of managers will give up that control, even if it is illusory.