Looks like new certifications have been developed that slot in between dental hygienist and dentist in terms of required training (independent practice dental hygienists and dental therapists). Neither can diagnose conditions themselves, though the latter can apparently drill and fill simple cavities under the supervision of an actual dentist. This hierarchy fits in pretty well with medicine, where there's nurse assistants, registered nurses, nurse practitioners, physician's assistants, DOs, and MDs.

And of course dentist itself isn't as high as you can get; these are procedures they won't do that you have to be referred to an oral surgeon for.

Anyway, point is, it's not nearly so simple as to just say that occupational licensing is extremely overdone in dentistry.