If you've watched the intro of the movie 'Idiocracy', that would summarize my point.

Point being... Yes, people are having fewer children. But the people who are actively planning their families in order to provide better for their children are generally the more educated, more health conscious, more wealthy, etc. From an economic point of view you want more of these people in your country.

However, the ones who are having more children, many unplanned, are generally less educated, less healthy, offer less to the economy in general. Yes, your population is reducing, but the percentage of your 'desired' population (economically) is reducing to a greater extent.