If you are arguing that managers with technical knowledge of the work being done are wholly unnecessary, then you are necessarily arguing that all teams are doing optimal work all the time, and have nothing to gain from external feedback.
I call bull on that. There are several times where, after discussing some estimates with my developers, it turns out that the overall approach was inefficient. By pointing them in the direction of a more efficient approach to the problem, and leaving them to work out the details, and making them feel free to discuss other alternatives, we have been able to get stuff done in reasonable time.
Your whole point of view seems to suggest some kind of conflict between technical managers and developer teams, where I see none necessarily. Managers are a resource for the team, not people who lord it over their teams.
I have been contracting and consulting for years. Without exception, the worst managers I've had in terms of demanding the impossible, and failing to appreciate the work I have put, were those without any clue what it takes to do what I did. The technical managers would work with me to write specs that made my work a breeze.