A lot of high level runners have movement flaws such as valgus knees, overpronation, arms crossing midline. Because these aren't "flaws". They're normal variations. Posture is a habit, not an ideal. Breaking the habit with more movement options is probably more useful than telling someone there is a "good" or "bad" posture. Expectations can shape pain. If we set people up to think that prolonged sitting or sitting in a certain position will cause pain, it will be more likely to do so. If ergonomics worked, we would not see any neck or back pain from office workers. If postural ideals worked, those in the army standing at attention for hours would have zero pain.