Carter was enough of a politician inherently that he probably would have made flag rank, but chances are he'd have retired sometime in his mid 60s (say, about 1989) with a lavish banquet...and then would have been largely forgotten within a couple of years. He'd be spinning out his retirement probably in northern VA (I doubt he'd have gotten outside the corridors of the Pentagon much), probably working on Habitat for Humanity projects, teaching Sunday School, and the like, in near-complete obscurity.