I don't think they tried to replace them, but just sort of give some modern alternatives that fit with the game elements they could provide in base game, especially as some jobs came later in the EPs. Secret agent is basically CIA, so they could be anything while doing their job. Also, technically speaking astronaut doesn't replace military career, because there are two types of astronauts, scientists and those who actually fly the rockets are usually fighter jet pilots. In TS4 rocket science goes with the scientist career and there is military career that came with Strangerville pack but it doesn't involve rockets as far as I know.
