Those hold on to the view of a "we are an already developed country, so we no longer need to advance" will be beaten by those who fight for a better future.
I have no idea why you think this is what is going on. This is
not the attitude that is going on, and "reproduce as much as possible" is not equivalent to "fight for a better future".
The reason people reproduce less now in developed nations is because they
don't need to. They don't need kids to take care of the farm, there are
more options in their lives to begin with, making kids less of a foregone conclusion, and the extreme decrease in childhood/infant deaths from illness means that if you only have two kids, odds are they'll both live... not to mention advances in birth control, which again, ties into the fact that having kids is less necessary to participate in society and the economy these days.
Even if humans colonized nine more Earth-sized planets and spread out evenly amongst them, that's still 600 million per planet. That is still a perfectly adequate number of people. Hell, if anything that would be an improvement. Greater numbers are not necessary, period, and expansion through massive reproduction has not been at all necessary for a very long time, and will not be necessary for a very long time.