There is the one-off about this being the last of its kind space vehicle, and the booster pack in orbit had been there for quite some time. You can somewhat fanwank that into meaning they had to use older, simpler technology to get the capsule itself into orbit, but then could take advantage of the pre-Blight technology already in orbit for the rest of the movie. That's about the same as your explanation, though.
Pretty weak, so it would be nice if they had at least given us a throwaway sentence or two to wave it away like they did with spaghettification, as the rocket orbital mechanics, at least, were distracting. I imagine the real reason was Nolan wanted to invoke a parallel to the Apollo missions in the liftoff from earth, but didn't want to waste time with the very hard problem of getting off other worlds once you get there.
Pretty weak, so it would be nice if they had at least given us a throwaway sentence or two to wave it away like they did with spaghettification, as the rocket orbital mechanics, at least, were distracting. I imagine the real reason was Nolan wanted to invoke a parallel to the Apollo missions in the liftoff from earth, but didn't want to waste time with the very hard problem of getting off other worlds once you get there.