Sorry but you are incorrect on at least this issue.  Not all dental or dental-related diseases are preventable.  Re-read the article and you will see the author's condition is Not due to lack of brushing and flossing.  Look up Pemphigus Vulgaris, for example.  NO known cause (so not 'preventable'), No cure, and treatments greatly vary in effectiveness. Existing on just soup and smoothies, shuttled between periodontist and dermatologist for care. Patients require visits every 60 days, if not more frequent - that is a lot of out-of-pocket costs for anyone with any insurance coverage. And thousands suffer from this condition.
Overall dental insurance is decades behind in what is covered. 1 exam and 2 cleanings per year is insufficient for a majority of patients.  And the prices for what is covered is also extremely out-of-date and not at all reasonable for what treatments actually cost.  
A previous insurance company needed to price my cleaning by 'how many teeth were involved' - the entire mouth/all of them -but no, they required a total number!  charge calculated per tooth?! Then I realized they were based in Kentucky - number of teeth must vary greatly there.