Most of the world doesn't believe in Christianity, though. Non-Christians don't just reject the Trinity; they reject the very foundation of Christianity in the first place and reject the ahistorical, academically unevidenced belief that Jesus rose from the dead. Even scholars and historians who believe that Jesus was God don't try to argue that their belief is historically evidenced, unless they want to open themselves up to an onslaught of criticism and subsequent lack of credibility.
Is rejection of the belief that Jesus is God equivalent to saying that Christians throughout the past 1,700 years didn't know what they were talking about? Conversely, is saying that Jesus is God or that there's a "New Testament" also a statement that Muslims in the past 1,400 years or Jews in the past 2,000+ years didn't know what they were talking about?
I don't see a reason to treat differences in individual belief under the umbrella of one religion in a significantly different way from differences in individual belief across different religions. In that light, non-Trinitarian interpretations of Christianity are, to me, as much of a valid variation in personal belief as the variations between dharmic and Abrahamic religions are, or the variations between each Abrahamic religion and the others. If we strictly stick to logic and historical evidence as the standards by which to judge theology, it seems to me that it's quite arguable that the entire belief in Jesus' divinity will have a difficult time passing muster too, not just non-Trinitarian varieties thereof. I see theological beliefs as too complex and diverse for those two metrics alone to fairly or accurately assess whether they're "good" or "bad," though—and the same goes for using tradition or prevalent belief as a metric to assess theology as being either.
Unlike with treatments for illnesses (as in the example you gave) that can be measured against observable results, I don't think there's any testable or measurable standard for determining whether a belief about a strictly theological matter is valid. It ultimately comes down to a combination of interpretation and personal belief—just as acceptance of Christianity's premise of a "New Testament" does.