The war ended in 1975. That was three years after Mabus got out.
If he went in as an officer in 1970, his minimum enlistment would have been through 1976.
The RIFs were already in place at that time. He was not given the option of active duty for any period, only an 8 year Reserve obligation. If I recall correctly, he was a sophomore when he elected to commit to the program, and the draft ended shortly thereafter.
One of his classmates had a full ride ROTC scholarship and he was RIF'd too. Reserve duty only.
If the Army was drawing down then, I'm sure the Navy was too, and probably sooner.