If you grasped population demographics and agriculture based economics versus current economics and demographics, you would also grasp why the Holy Spirit guided the Church to this knowledge at this time in history. I know there is no such thing as overpopulation. But I also know there are real concrete circumstances when NFP use is morally licit, and rare circumstances where NFP use verges on a necessity (Catholics in China who will have their baby forcefully aborted.)
NFP is not immoral if used in grave circumstances. I have seen these grave circumstances first hand, both at home and abroad. That is why I bristle at those who condemn NFP use and NFP users, when such are NOT condemned by the Church, if grave circumstances for recourse to NFP exist.