Fair or not, the issue of divided loyalties is viewed prospectively, based only on the circumstance of birth. We don’t know how any individual will turn out, 18, 30, 50 years later. The test is at birth, until we open the office to naturalized citizens, then it becomes whatever/whenever.
Ted Cruz’s birth certificate proves without any doubt that he was a Canadian citizen at birth with the right, because of his mother’s US citizenship to become a US citizen if the appropriate paperwork was filed.