A 2020 Cochrane review that assessed the two clinical trials concluded that "whether adults see their dentist for a check‐up every six months or at personalized intervals based on their dentist's assessment of their risk of dental disease does not affect tooth decay, gum disease, or quality of life. Longer intervals (up to 24 months) between check‐ups may not negatively affect these outcomes." The Cochrane reviewers reported that they were "confident" of little to no difference between six-month and risk-based check-ups and were "moderately confident" that going up to 24-month checkups would make little to no difference either.
Likewise, Nadanovsky and his colleagues highlight that there is no evidence supporting the benefit of common scaling and polishing treatments for adults without periodontitis. And for children, cavities in baby teeth are routinely filled, despite evidence from a randomized controlled trial that rates of pain and infections are similar—about 40 percent—whether the cavities are filled or not.
I think the 6mo cleanings are the prepare for people who don't take good care of their teeth. My plan allows for 3/4 cleanings a year if I want them. And some people are a lot more prone.
What?
Genetically, some people have weaker enamel. Economically, your childhood dental care will greatly affect your adulthood needs.