Good try, but you forget the character of English general practice, where the vast majority of my work was done. My partners and I looked after the health of 12,000+ registered patients, including being the NHS gatekeepers for all referrals to secondary or tertiary care.
With the exception of those I killed who couldn't tell tales, or those so angry that they left the list without explanation, you can be sure I got plenty of feedback from my colleagues, as well as from patients seen in many different contexts. Most of my failures were, naturally enough, referred on by myself on routine review.
Only in the last two years, when I ran the district back clinic, did I lose that continuity, but it was some compensation that I could now refer patients to myself (in order, of course, to access the diagnostic and treatment facilities available via the clinic).
The way that clinic turned out as we developed it, it became mostly a chronic back pain facility (cynically, I would suggest, largely because so many patients had received such inadequate initial care from conventional treatment). But as I think about it, a good number of patients referred from the district ended up with a trial of manipulation first, and many responded to the extent of not needing extra treatment. Naturally they received open appointments to return if necessary for the next step.
Also bear in mind that I was not a one-trick osteopath or chiropractor, but a qualified doctor, using, or referring for, all the conventional treatments as I found appropriate. Anything you say about confirmation bias applies equally to every prescription I ever gave, and every session from my in-house physiotherapist. Evaluation by myself, and my partners, and the patients of the "hands-on" treatment was in that context, as about 15-20% of my workload.
I should add that, coincidentally, another GP in my city had learned the same techniques from the same physicians, though our techniques had evolved interestingly differently over the decades. We ended up running a training course in manipulation together. As it happens he was the guy who organised the academic program for the County Medical Committee.