As a leader I would never institute a one-child policy, which I consider to be an immoral restriction on personal liberty. But if we ask whether this policy had benefits for China, it absolutely did.
For instance the policy made China a more educated society more rapidly. It is simple economics that putting a lot of money into the education of each child is easier to do with a single child than with three or for that matter seven kids. The effects of the one-child policy are illustrated through a natural experiment of sorts. Chinese children who ended up born into twin pairs showed significantly slower rates of schooling progress, worse grades, lower chances of college enrollment, and worse health. These differences do not follow mainly from the lower birth weight of twins or other birth-related problems (though that is one factor), but rather they stem from the lower resources which are invested in children in larger families.
By the way, the one-child policy was not the main reason why Chinese fertility fell. Between 1970 and 1979, before the policy was put in place, the total fertility rate fell dramatically from 5.9 to 2.9. After the policy was introduced, the total fertility rate actually fell more gradually than during that earlier stretch, settling into 1.7 by 1995. The best estimate we have is that the one-child policy lowered Chinese births by an average of 0.33 per woman, which is a noticeable but not drastic change.
Even in purely practical terms, it is highly likely the policy has been obsolete for some while.
See Therese Hesketh, Li Lu, and Zhu Wei Xing. “The Effect of China’s One-Child Family Policy after 25 Years.” New England Journal of Medicine, September 15, 2005, 1171-1176, and Marjorie McElroy and Dennis Tao Yang. “Carrots and Sticks: Fertility Effects of China’s Population Policies.” American Economic Review, May 2000, 389-392.