It is a very important factor. After all, all economic value come from labour, not to mention economy of scale and division of labour, which are more efficient with higher population. I really doubt you can get space-faring civilization with 500 million people, satellites and unmanned exploration would be its limit at best.

Also, I fear a scenario, where the world collapses back into agrarian traditionalism, because it is the only known way to sustain population and all the other societies just decline into irrelevance, and then we get another cycle of class society, until we finally manage to solve this problem.