I'm not what your point is exactly.
But anyways, the best solution for our world problems is more economic growth. Economic growth equals less population growth (because there are more things to buy- better quality of life- and since human life is more complex, it's a competitive advantage to focus on fewer offsprings- or maybe there "some" other reason but prosperity has resulted in negative growth rates in the richest "developed" countries- e.g. it could simply be that women have more power and don't see bearing children as the major reason to exist). Economic growth equals less people who are forced into a situation of not having enough food or housing. Humans are valued- you have fuller employment and employers need people and pay them well. Economic growth means you have competition which forces managment to manage better. It also means more diversity- in all things, including such ideas as having a healthy environment (clean air, water, low pollution, wildlife, ect).
You get economic growth by getting more customers (producing more products and selling more of your products than you did the year before). There are a number of ways of doing this, but one way is by lower your costs so that your product is more affordable (or it's a better product- worth more, but at same price).
The advantage I see of space development is that it will lower costs. If I thought space development wouldn't lower cost, I wouldn't favor it.
At current launch costs it may be difficult to see how the space environment can lower costs- even though there are currently some products which are cheaper because they use space (e.g. communication satellites).
Current launches are hideously expensive, but this cost can be significantly lowered and as this happens more things are going to cost less to do in space than compared to doing them on Earth. This will increase demand for more launchers and market forces will lower costs further. Once space development has matured, most manufacturing currently done on Earth will be shifted into space- space environment will allow cheap energy and most manufacturing uses lots of energy. And there is no shortage of mineral wealth in space. So at some point in the future it won't be very profitable to mine or manufacture on Earth because it can be done cheaper in space.
So space offer a way of getting economic growth without becoming more dependent on third world countries and Middle East oil, and shifts heavy industries off planet.
I ask you, "What else could a fanatical environmentalist want?".
This is much easier than trying to make some people not want to have children and/or wanting to kill people.