The people who debunked these theories were quite familiar with these existing models and how to build rigorous tests to differentiate these cases from new ideas.
The study tells us that most detractors are not people like this, but instead people with below-average understanding of the field and related fields-- and who are willing to bet more on their performance of objective tests of those fields than the non-detractors.
Sadly, the spherical “scientists” in a vacuum rigorously and unselfishly digging to get closer to the “truth” is only a fantasy found in school textbooks. What is called “science” has nothing on “truth” or “reality”, it's a specific method of creating virtual models that are many many degrees less complex than the “real thing”, and then treating real thing as if it was its model to achieve some outcome. In some way, that's how humans adapt to environment, and change it. Even a commoner can read 18th and 19th century works that spread those stereotypes on “Reason”, “seeking the truth”, etc., and see how rosy the initial reasoning was. Despite the fact those sources are long forgotten by the public, the status quo of everyone's education has been formed by them.
Phrenology was not an “error”, “bad page”, yada yada on the Glorius Path of Progress. It was The Science. (Well, maybe some “overconfident” scientists disagreed with it for personal reasons, but they were “not representative”.) It was evident that all those unfamiliar people from far away lands, and all those lower class social parasites were less developed than the Educated Man armed with Scientific Knowledge, and if was perfectly reasonable to try finding the source of such a pitiful condition inside their skulls.
It shouldn't surprise anyone that multiple generations taught to think like that resulted in people casually talking about races and signs of degradation at dinners or, well, political rallies. They were given that model as truth.
By the way, archaeologists and biologists still use observations and concepts from phrenologists without ever “cancelling” them. Squint your eyes one way, and it's “pseudoscience”, squint them the other way, and it's “science”.
Or the idea that makes any pop-sci nerd twitch and giggle in anticipation: so there is no success in finding the material soul, but maybe we can study what that soul thing does by looking at its interactions with material world, and using scientific method? Let's call that… right, psychology.
Your response is completely orthogonal to what I said. I'm not too interested in hashing out the basics of the philosophy of science for the purpose of pedantry on the internet. That said--
Specific theories and predictions of phrenology were debunked and discarded with evidence. That happens to make almost all the utility evaporate, but that's not to say there's not quite any left.
Of course, there's a whole lot of predictions and data from alchemy that became the body of knowledge of chemistry, too, even if a lot more evaporated.
The other thing to point out is that the overall scientific method itself has evolved and is still evolving. Debating various kinds of errors and their nature made in the 18th century when even early 19th century practice was sufficient to refute them is not likely to be productive.
The study tells us that most detractors are not people like this, but instead people with below-average understanding of the field and related fields-- and who are willing to bet more on their performance of objective tests of those fields than the non-detractors.