I don't disagree with you sentiment about children that could do with help but parenting is a complex issue both biologically,emotionally and socially. Its not case of simple substitution of the biological withe adopted.
I think its a sign of progress that we are able to do this. In the same way that any medical advance can improves the life of an individual. Just before Christmas, a statement was made in the news that there are more women now with narrower pelvises having children; which means natural birth is more difficult/unlikely. It was even followed up with most women in this category would die in childbirth. Whilst these things maybe true...I'd rather not have folks die if we have the means to save them.
I do disagree with the notion we have "far too many people in the world". I haven't seen any evidence for that. I mean how many is too few or too many? The only fact we can see is that we have more people alive than we have in the past and its possible to make the hypothesis that populations aren't distributed in the most effective and efficient manner uniformly across the world. I.e. high population density in some areas causing a strain on available resources or low density in other areas the same albeit in a different manner.