Secular, yes. Western and liberal? No.

But you can't just look at population, you have to take the derivative, maybe even the second derivative. What's the population growth rate, and how is that rate changing? That describes future demography. Whether you're currently overpopulated or not is just the status quo. It's the past.

Because of their one-child policy, China will also have an aging problem in 30 to 40 years time. Theirs may even get more of a problem, as their replacement rate may be (Chinese statistics are inaccurate, so it is hard to tell) lower than that in Europe.

I remember hearing that China's one-child policy only applies in the cities, not in the country. If China's rural population urbanizes they won't have a demographic problem--in fact, the one-child policy explicitly makes room for the rural population to urbanize, which is a big component of development and industrialization.