The idea itself isn't that controversial - age of majority laws for contracts, voting, and even medical treatment clearly acknowledge the idea that some people are too young to know what they are agreeing to, so the decision should be made by someone else who takes responsibility for them (most things), or there should be no decision because there doesn't need to be one (voting).
We just need to refine our understanding to reflect the reality that as old as you may get, you will never understand some things.
Without autonomy, the concepts of voluntary/involuntary lose meaning, which can enable some very nasty scenarios. Exceptions to autonomy are rare and come with stringent legal duties and obligations on the responsible party. Doctors are not responsible for their patients in that way; they are service providers.
Lack of understanding should not be sufficient to undercut an ethical principle like autonomy. Imagine if I had the power to decide that you don't sufficiently understand the concept of autonomy, and must be shipped off to a re-education center.
Talking about it outside of the specific conflict, is disingenuous. Everyone understands how important Autonomy is. That's part of the issue, not a supporting argument to value it over the health of a patient. If there's no patient, there's no Autonomy either.
So, I do not agree with you that "everyone understands how important autonomy is."
You don't see the irony in that, when talking about the Placebo effect? This discussion is about an implicit assumption.