One of the major points of the "basic income" movement is that technology is supposed to let us work less. In this case, Dentist B could just leverage the knowledge of other dentists as posted on the internet. Or we would devise more effective professional development programs. Or Dentist B would quit altogether, and we could build remote dentistry technology, allowing Dentist A (and others like her) the ability to service far more patients. Etc. etc. etc.