As for the truly unwanted ones, ~33 million couples in the U.S. are infertile. There are untold thousands of families waiting to adopt. A few of them are listed at https://www.americanadoptions.com/family_profile/browse
As for me, I'm raising several children. My wife and I have considered adopting on top of that. I was adopted.
Steve Jobs was adopted. So was John Hancock. Nelson Mandela. Leo Tolstoy. Ray Liotta. Simone Biles.
In terms of economic and societal impact, I'd wager somewhere in those 60 million missing children, there were a few hundred kids that could have grown up and made a real positive impact on our lives. We'll never know how many diseases would have been cured by now, or Olympic medals won. Millions of lives. Just gone.