I cannot speak for all historians, but I can tell you how I approach this problem in my work. First, I don’t consider historical actions even in seemingly odd cases as instances showcasing “less intelligence”. Hegel’s famous dictum (‘The owl of Minerva spreads its wings only with the falling of the dusk’) reminds us that the subjects we investigate do not possess the same information about the consequences of their actions as the temporally distant historians. Considering historical subjects as “less intelligent” because we now have more information about the consequences of their actions is a type of presentism - that is to say interpreting the past based on what we deem as contemporary norms, values, and practices. (Spoerhase 2008)

Second, your reference to Pliny is fascinating in this context. His Historia Naturalis, in which the “cure” you mention appeared, was an encyclopedia of treatments. However, not all of the treatments in his book were considered useful by him. Instead, his aim with the book was to demonstrate the “follies” of non-Roman herbal medicine. It is useful to think about it as a book denoting the scandals of individuals practicing non-Roman herbal medicine. For example, he tells the story of Archatagues who gained the nickname “the executioner” for the “violence in cutting and burning” involved in his treatments (Nutton 2013: 164). With this in mind, the example that you mention is actually being recommended in Pliny’s book by a character labelled as the Magi. Pliny deployed this character as a stand-in for the “foreign charlatan” deceiving Roman citizens. (Dykstra 2007) Therefore, it was not a treatment recommended by Pliny, but one that was recommended by an “antagonist” in the book.

In a way, just like you care about individuals in the past receiving the right treatment, so was Pliny. Historia Naturalis would give you the answer that the success of these physicians (or “charlatans”) and their non-Roman methods was due ‘to the gullibility of [...] Roman patients and the power of [“the charlatan’s”] rhetoric’ (Nutton 2013: 171). Yet, despite Pliny’s insistence on the “right” type of medicine (i.e. Roman herbal medicine), Prioreschi argued that ‘a return to the old ways of treating disease [i.e. Roman herbal medicine], as [Pliny] seems to advocate, might have not produced better results’ (Prioreschi 1996: 230). Or to put it differently ‘[i]n terms of effectiveness and closeness to medical reality, magic and naturalistic approaches were not far from each other’. (Prioreschi 1996: 224) So my fascination with the inclusion of the specific example from Pliny in your question is that the treatment (1) was not prescribed by Pliny, (2) he objected to that actual treatment, and (3) he was to a certain degree concerned with the same problem (“the follies”) as you are. Thereby, here’s a contemporary of the “less intelligent” ancients calling his contemporaries “less intelligent”.

“Presumably the first time, or at least the second time, somebody tried this and their tooth didn't feel better, they would have stopped.”

Third, I interpret what you mean by your last sentence as there must be cases when the effects of a treatment are immediately observed, which should convince the individual to deviate from their previous actions. This point is also indirectly implied by Prioreschi’s statement about the effectiveness of Roman herbal medicine. It is interesting to frame the question of effectiveness through the lens of the history of emotions, and more specifically through the history of pain. Pain is not only a personal experience, but it is also something that has to be identified by others (especially during treatment). This need to define pain as a category makes pain dependent on its historical context. (Boddice 2014) Similarly, our own personal experience of pain is historically situated. This is especially the case when we consider religious contexts, where pain can be interpreted through various different (e.g. sharing Christ’s suffering) (Boddice 2017). This also brings up the question of power-dynamics between patients and doctors, and how they changed over time, but that would lead us to the sociology of medicine (I’ll add a couple of books about it in the readings) (Lupton 2012; Bynum 2008). In brief, an encounter between two individuals to treat a toothache (or any other pain) is complex and is defined by the historical context.

Finally, I also sense in your question a little bit of allusion to issues of trust in relation to science and medicine. In the case of Pliny, it was also a question of trust in medicine. He was trying to question the trust placed by the Roman public in the new medicinal practices, and recast Roman herbal medicine as the only trustworthy approach. With this question in mind, you might enjoy reading ‘Why Trust Science’, which is a recent book by Naomi Oreskes (2019). In the book she tackles the question of how we have moved on from debates about value-free science, and therefore, tackling issues of trust in science with still that initial assumption is less effective nowadays. Instead, one of her recommendations is to encourage scientists to talk about their own values in order to demonstrate that their interests align with the people. According to Oreskes, this would help in removing the obstacle of the assumption that experts and scientists work for the “other side”/”enemy”. The requirement to include funding sources in published research papers is one step that has been adopted over the past years with this approach in mind. Of course, no approach is bulletproof, but to elaborate on that we would need another question.

Sources: Spoerhase, Carlos - "Presentism and precursorship in intellectual history." Culture, Theory & Critique 49.1 (2008): 49-72. Boddice, Robert (ed.) - Pain and emotion in modern history. Springer, 2014. Boddice, Robert - Pain: A very short introduction. Oxford University Press, 2017. Dykstra, Sarah Sophia - Portentous fantasies: Pliny's representation of the Magi in the Historia Naturalis. MA Dissertation, University of British Columbia, 2007. https://open.library.ubc.ca/cIRcle/collections/ubctheses/831/items/1.0100728
Nutton, Vivian - Ancient medicine. Routledge, 2013. Prioreschi, Plinio - A history of medicine: Roman medicine. Edwin Mellen Press, 1996. Oreskes, Naomi - Why trust science? Princeton University Press, 2019. Lupton, Deborah - Medicine as culture: illness, disease and the body. Sage, 2012. Bynum, William - The history of medicine: a very short introduction. Oxford University Press, 2008.