In South Korea, there is a bit of a gender imbalance too, though not as severe, even without the 1 child rule. High education costs and traditional attitudes towards working mums in South Korea have resulted in the one-child family becoming the norm there as well. And as long as that is the case, lots of families will want that one child to be a boy.
I suspect that the oversupply of boys in China will continue for quite a while--but as the Chinese grow richer, it will increasingly take the form of selective abortion rather than abandonment of infants.
I think once a society has adapted to very low birthrate, it is difficult for the birthrate to go higher again.
Families get used to the idea of having more disposable income, and don't want to give it up. Also, societies start to get onto the intensive-education treadmill where loads of time and money and effort are invested in getting your child to succeed educationally. This results in high rates of academic attainment (as seen in Shanghai and South Korea), but also makes it hard for parents to have more than one child. Private schools and tutoring centers inflate their prices because they are working on the assumption that parents of a single child will have more cash to throw around per child.