"The Agencies" actively recruit junior officers departing active duty: college education, security clearances, active lifestyle, concept of duty, fitness for goverment service -- exactly what they are looking for. I've known several Navy and Marine O-3's go to the FBI, CIA, Foreign Service, Homeland Security, etc. A career as a military officer is a solid foundation for civilian careers in both government and business.
The military manpower system is designed to accommodate shedding people along the way, SA's included. Over 1000 will come in on I-Day, and peel-offs begin that summer and continue for various reasons until graduation. Once on active duty, most will complete their service obligation. A certain percentage are expected to move on to other things after year 5 or whatever the particular obligated service point is, depending on warfare specialty. The winnowing continues at every promotion juncture; the number of officers at every grade level is controlled by law. Manpower modeling analysis underlies all the staffing and promotion decisions in the services, determining everything from intake numbers at commissioning points to promotion numbers each year.
That said, since USNA and the subsequent obligation will take up almost the next 10 years of your life, your primary focus should be on serving in the Navy or Marine Corps with commitment and desire. Be open to all the ways your military career could unfold, including a long and rewarding career in uniform. Retiring after 20-26 years of active duty, you are not teetering on the edge of the grave just yet and can go on to another career as a civilian. At the point after your initial obligation where you evaluate staying or going, junior officers have a range of employment opportunities.
It's good to have mid and long-range goals, but don't look so far ahead you don't clearly see the path most immediately in front of you. Dmeix' post is on point.
Lastly, the mission of the Naval Academy. "To develop Midshipmen morally, mentally and physically and to imbue them with the highest ideals of duty, honor and loyalty in order to graduate leaders who are dedicated to a career of naval service and have potential for future development in mind and character to assume the highest responsibilities of command, citizenship and government." The focus is on producing leaders of the highest caliber who have the potential for a career in the naval service, but the window is open for continuing that leadership out of uniform.
Just MHO... always good to have dreams and look ahead while being firmly rooted in the present.