> when you say "Low-end Scrum" is just the only type of widespread Scrum we've ever seen practiced anywhere, ever, to the point that it makes us question whether Scrum itself is partly responsible [...]

Yes, exactly. I know some good people who are dedicated Scrum fans. In their view, the engineering practices are all implied, because if you do have a relatively autonomous team dedicated to continuous improvement, you'll get there. And it works, because they are smart and dedicated people committed to doing the right thing.

So in that sense, the idea of Scrum is a good idea. But Scrum-the-movement and Scrum-the-business have fallen very short of that ideal. So in practice about 98% of Scrum implementations I've seen make me despair for humanity.

> What I really wish Scrum or Agile had, was a more unbreakable way to let development teams say no

One interesting version of that was the Extreme Programming Bill of Rights. One version of it is here (and in XP jargon "customer" is something like "product manager":

http://www.agilenutshell.com/bill_of_rights

It's not perfect, but I think it was a good attempt to make clear that there's a useful balance of power.