Unfortunately, I do not know the answer to this. I only have the current roster to reference. It sounds that, at least a couple of decades ago, there were significantly more graduates in the company. I think your point about the two organizations not being officially affiliated (and therefore the same level of “interest” not being there) is sound.
Ultimately, the fact that NBS graduates are dancing in world-class companies around the world speaks for itself.
Being more familiar with San Francisco Ballet I can definitely see what you mean about the time it takes to nurture home-grown talents. It does not happen overnight but it is possible. And what a treat for an audience to witness such development and growth.