In 1789 (the year after the Constitution was ratified,) Dr. David Ramsay (<=click on link to see who that is) published an essay entitled "A Dissertation on the Manners of Acquiring the Character and Privileges of a Citizen," a very important and influential essay on defining a "natural born Citizen." In his 1789 article, Ramsay first explained who the "original citizens" were and then defined the "natural born citizens" as the children born in the country to citizen parents. He said concerning the children born after the declaration of independence, "[c]itizenship is the inheritance of the children of those who have taken part in the late revolution; but this is confined exclusively to the children of those who were themselves citizens..." Id. at 6. He added that "citizenship by inheritance belongs to none but the children of those Americans, who, having survived the declaration of independence, acquired that adventitious character in their own right, and transmitted it to their offspring..." Id. at 7. He continued that citizenship "as a natural right, belongs to none but those who have been born of citizens since the 4th of July, 1776..." Id. at 6.
So a historian in 1789 gives us the definitive explanation of who is a "natural born citizen" but our elected representatives are forbidden to do so. Got it.
I think you're missing Buckeye's point. I agree that the power to naturalize doesn't mean the power to revoke the citizenship of native citizens, but I've yet to hear anyone argue that it does so I don't know why you keep bringing it up.
Congress's authority to grant citizenship to non-citizens in no way threatens existing citizens, but it does help to define who the "naturally born" ones are - those who don't fall under the naturalization statutes.