It worked well for us for purely selfish reasons. If they hadn't done so, I wouldn't have much family as an adult as we have no extended family.
Climate change- yes, we only have to look at highly populated China to see what damage they do as a whole even though they actually cause LESS damage per capita than countries with a lower population than theirs like USA, Germany, Qatar, Canada and others. So it's not only what we are doing but how many of us are doing it.
BUT, probably a whole other thread but it's been shown that those in poorer countries might have more children on average but that they do less damage to our planet whilst also feeling the worst effects of climate change caused by the people in richer countries with less children, so swings and roundabouts really and you'd be banning the ones causing least damage from having 3+ children because of the actions of the ones who have 3 or less if you did make that law. Over population isn't good but I think there's more to it than that.
I wouldn't personally have so many. Then again, I might think differently and do what my parents did if I knew mine would have no cousins, aunts or uncles. Who knows!
I do see your point though and do worry about the way we farm, way we fish, deforestation etc. I do also believe there are other more sustainable ways to live without causing all this damage but consumers wouldn't like it and the big companies wouldn't like it so it won't happen, though younger generations seem to care much more about these things than previous generations have so it could change.