I got an interesting compliment from a brown belt student a while back.
Regarding why we place so much emphasis on the white and yellow-belt material, he told the class, "I haven't learned any new throws since about yellow or green belt!" That's not to say that he has stagnated since green belt - just the opposite! He has seen all the material up to about shodan level and he has tried it and can do it just fine - because we spent so much time and attention to the white and yellow-belt material.
Everything after green belt is just minor tweaks or modifications of angle or timing. The most important core of judo, we teach at white and yellow belt, and it sets the foundation that makes the rest of the syllabus almost trivial.
That is also not to say that you can do 9 months or so of judo and get a green belt and be done. While you will definitely have the foundation by then, you will still need to learn how to properly make those tweaks and modifications to the core principles in real-time with a live opponent - and that is what you spend the rest of your lifetime in judo doing - filling in the corners!