Yup. I loved the Manifesto. They had the right idea.
My experience is that things go pear-shaped when those pesky "people" get involved. It's all great on paper, and in the theoretical perfect world, but as soon as people start getting involved, and developing their own mental models, it can cause problems.
A big factor, as well (IMNSHO), is that a lot of folks decided that they wanted to be "gatekeepers" of the methodology, so they started to formalize it, and develop a vocabulary that they "own."
That happens with pretty much everything.
I'm old, grumpy, and very experienced. A great deal of the programming that I do is "instinctual." I don't stop and say "This would be a great place for OBSERVER!". I just go ahead and set up a subscription management utility, and don't give a damn what it's called.
In my experience, every time we come to a "new paradigm," a great deal of it is really just a formalization or rebranding of concepts that have been around for years.