AP Exams Continue to Evolve - And Run the Risk of Rendering the AP Curriculum Superfluous

This very interesting blog post about the soon-to-be-unveiled changes to the AP U.S. History exam demonstrates the commendable evolution process that AP exams are undergoing, much to the credit of the College Board.  These changes do, though, raise an interesting paradox; if AP exams become (as I ardently believe they should) more heavily focused on critical thinking skills and the habits of mind required in the particular academic disciplines, why would it be necessary to have a prescribed, and by definition constricting, AP curriculum?  Wouldn't any highly challenging course that developed these skills and habits of mind accomplish the goal of preparing students to succeed on these tests and - far more importantly - indicate their readiness for more advanced study in the field?  It speaks well of the College Board that they are willing to evolve their way into such an existential conundrum.