Ever since the first Ironclad appeared, there have been those who have argued that the Marines should go the way of the clipper ship. The institution of the United States Marine Corps itself has been in an existential crisis in Washington DC ever since.
This author is of the opinion that the job of the Marines has in fact, never changed; only the circumstances. The first Marines slept between the officers and the crew; ever vigilant.
Duty, Honor, Country are words that are up for interpretation by individuals. The Constitution is not. The Prime Directive of the United States Marine Corps should be to defend the Constitution of the United States of America. Specifically, the Bill of Rights and other amendments.
If I had my way, every Marine would know the Constitution as well as they do their rifle.