Massive migration to space : a myth
For several reasons, huge number of people migrating to other worlds, won't happen....
Only poorest nations will have a motivation , a push, to leave the planet, but they won't have the technology to do it.
Those who would have the technology, are too well off , too old and least motivated to leave Earth.
And world population might well shrink (after reaching a peak of 9 Billion; I've heard that if the trend continues, in 400 years there won't be any human beings left at all, not just in space but on Earth too).