The brevet system was done away with after the Civil War and for the reasons that you have illustrated. It must have been hard on the egos of most of these CW veterans to have reached a brevet rank only to find after the war to be reduced in rank well below that brevet. Another example of the US Army of the time trying to do things on the cheap - raise a huge army and officers then eliminate it after the war (as we typically did) and then those officers that were to serve after the war against the plains Indians is to reduce their grades to regular ranks so they cost them less (being paid for the lower rank). The British system for officers out of war was to keep them on the rolls of the army but on half pay.
Greg Biggs