The 'claim' of it helping the environment is a bit stupid because those people often will want Africa, Asia etc to reduce numbers yet those populous areas particularly Africa puts less strain on the environment compared to the Europeans and Americans(in particular). In fact if the American population reduced their consumption of resources alone as population, the 'claimed' need for population reduction goes out of the window fast. Of cause Americans refuse to reduce their consumption and see it as their 'right' to do so.
2ndly problem with Africa is bad management and use of resources available. Solve that and Africa will become completely independent of others more rapidly.
I think its inevitable that other population groups will soon be more prevalent that the 1st world and the next world war as such is likely to be either one against western imperialism or a fight amongst themselves over control of other countries.