Back in the Bush administration there was a flap related to the Army's publishing of its manual on interrogation techniques. Some argued it was wrong to described permitted and prohibited methods in detail, because it would enable terrorist organizations to train their members to resist interrogation. That seems to make sense: we can't exaggerate how wily and tricky these terrorist networks are.
But if that makes sense, then surely there's no need to modify the Miranda warning and law with regard to U.S. citizens and residents. Any smart terrorist organization understands that these people have rights under the Constitution, rights which aren't dependent on the Miranda warning. Any libertarian will tell you there's no obligation to say anything to a law enforcement officer. So smart organizations will train their US citizen recruits in their constitutional rights, and modifying the warning will do nothing.
So there's a choice: believe in smart terrorist organizations and don't change Miranda; or, believe terrorist organizations are less than smart, that Murphy's Law operates there as well as elsewhere, and the terrorist threat becomes too small to warrant any changes.