Well if she finds having children not to be attractive, that's fine. But I doubt that further decline of birthrates in the West will stop overpopulation.
Overpopulation isn't ultimately the problem, but rather over-consumption of finite resources. If people consume resources in an unbounded way, then population doesn't matter. The energy consumption of someone in the US is roughly 30x that of someone in Bangladesh for instance. With advances in technology, particularly in automation, we will be able to industrialise to a greater extent, and consume more.