Not to mention, cavities and other dental problems pose a measurable threat to your health and longevity (not to mention quality of life), so I would bet a certain amount of radiation risk can actually be offset by the resulting improved dental health.

That's the wrong question. The question is if the X-rays are actually necessary in order to have good dental health, and apparently the answer is no.

Dentists use them because "what's the harm", but now that a harm is demonstrated they should use them only when actually necessary (when "the benefit outweighs the risks").

Um, a dentist's time isn't infinite, and other methods of examination can be more annoying for the patient than an x-ray. I guarantee you that if we don't use x-rays then in practice people are going to suffer more from dental issues.