Actually, having done my research (technically I got someone else to do it and am using his findings) the earth could sustain about 4 billion people at a modest standard of life. We have 6 billion at last count, and we are not living modestly.
We aren't curbing our breeding, the fact that the rate of childbirth has actually gone up since 1960 (cutting out the boom after WW2) shows that we aren't curbing it and our medical advances are keeping the infirm alive far longer than is wise.
While there is land available, who is going to volunteer to go and live in the nice open areas of Siberia or Alaska or Africa? sure, we have the space, hell, we could even manage with the resources, but we WON'T, and in order to balance out the greed of the world, we need to make sure that there are less people to be greedy. I'm all for saving everyone (nearly) and making us all less avaricious, but it's not going to happen, so plan B is in order.
Regardless of why the problem happens, it's still ongoing. Giving poorer countries more money will only exacerbate the problem unless we can get them going straight to renewable, clean, recyclable means of production.
I'm not ignoring the solutions, I'm saying that while they might exist, they aren't being used, and extra bodies on the heap is the only way to do it, because it's forced, rather than optional. because we have democracy, any push to force people into leading better lives is met with out and out rejection. The various car taxes put in place for the goo of the environment are (violently in cases) pushed aside and the government labelled as morons or worse, and then voted out. Humanity needs to simmer down and accept that it can't just grow and grow and take over everything, but we won't, so we should be forced to. Depopulation would slow expansion and resource demand, so it's a good thing.
Oh, and setting up colonies on other planets is just plain irresponsible until we can learn to be at peace with this one.