Training others is a difficult job. Captain Sobel might have been the best man for that job. A winning army needs different skills for different jobs: training, logistics, planning, and fighting, to name a few.
Hard, tough training can save lives during combat.
Might have been, for those times— quite clearly was NOT what was needed shortly thereafter, useless in true warfighting skills (as proven in later scenes, including his rightful removal from harm and proper placement as a bookkeeper).
They also serve who stand and wait.