I think that having a standardized, rank by rank curriculum is a more modern concept. back in the day, I think a system contained a body of knowledge and techniques and kata, but exactly when they needed to be taught was more a judgement call of the instructor, so this left rank promotion also more open to his judgement call.
Some arts are still done this way. My capoeira school is a good example. There is absolutely no standardized curriculum, and no kata in the way Asian arts tend to have kata, but there is a large body of techniques that we train in the system. Everyone can train any of the techniques, no matter what level they are at. Some techiques are essential for developing a strong base and basics, so these would be focused on more heavily with the beginners, but that doesn't mean the beginners can't do other things too. Once the instructor feels your overall development and use of your knowledge has improved sufficiently, you can be promoted. More advanced students are expected to be able to do certain things that a beginner would not be expected to do, but all levels can work on it together and it is not entered into a strict curriculum.
Sometimes people who have trained in a more structured system find this frustrating, because they never know what they need to know. But the truth is, they need to know it all, but the development is gradual, over time.