In some countries, the MD degree will allow you to make less than many taxi drivers. Thus, for instance, in Egypt, you find many qualified physicians driving cabs.
The trick of the American Medical Association has been to create scarcity along with demand by restricting numbers of medical schools and number of student slots. This has created pent-up demand which has been met by the number of foreign-medical-school graduates (FMGs)coming to America to practice and get rich.
Although FMGs participate in the system of artifically-induced shortages and gain benefits of inflated salaries because of this, ultimately they will probably overturn the system as the number of FMGs decreases the artificial shortage of American physicians.
Other graduate degrees, such as the MLS, would greatly benefit from a ruling body restricting the number of schools and graduates, imposing licensure requirements for all librarians to be able to practice, and somehow creating insatiable demand for degreed librarian services (difficult).
Could this be done, it would be the librarians with lucrative private practices, attentive staffs, new BMWs and the adoration of society.