I've got to agree there, but personally, I think that any idea of moving millions off-planet is simply science fiction and in my opinion, just isn't going to happen.
I dont think we humans will colonise space in an orderly fashion.
First we get cheap launches to space, this is the biggest step.
This allows for tourism, and larger cheap satellites.
As a result of this, we may end up with space hotels and lot more satellites in space.
Suddenly you need services, such as commercial water and food refining/ recycling depots in space which requires more people up there, but is cheaper than launching new stuff up every time.
Maybe there forms a business case to service satellites, this requires more people.
Someone figures out how to refine mined resources up there, more people.
Before you know it, there are hundreds - thousands of people up there, just doing day to day stuff.
This is all without colonisation efforts.
The most we might see is "seed colonies" of maybe hundreds, but that's so far in the future, that it may as well be science fiction.
Let's see if self-sufficient colonies on say, the moon, can work first, though there are probably more resources on Mars.
Maybe, I'm not far-sighted enough.
As for the earth, either the population is controlled voluntarily or it will happen catastrophically, either way, I won't be here.
Good luck!
There will never be permanent colonisation unless there is
a) a business case to send people there.
b) once there people are there (Moon, Mars or anywhere) they need to be able to sustain themselves without constantly drawing resources from earth, within a few years (20 or so) of arriving.
I think both conditions are not too far away.
Colonisation on earth happened for the same reasons.
Oh, and Military. If there is a military advantage of having a base up in space/moon/mars. Then suddenly colonisation speeds up. Those bases need a lot of support, and China is looking to push for Moon bases sooner than most will realise