Based on the known factors outlined in my previous message, birthrates will decrease until portions of the world where any significant number of people actually want to live (the key caveat missing in your description) are empty and/or space travel gets reasonably cheap, and then edge up toward equilibrium (if only the former happens) or regrowth (if the latter happens).
Everything is in a perpetual state of blind evolution.
Nonsense. It has been repeatedly demonstrated that wealth induces people to lower birthrates to sustainable levels (and that the opening of new frontiers induces them to raise birthrates again).
What induces people to lower birthrates is selfishness and birth control. I don't know any couples who based the number of their children on "the opening of new frontiers."
Define "sustainable."
As a libertarian, you should know that resources always become less expensive in a market economy and that people are a society's greatest form of wealth.
Julian Simon. People's ability to manage their own affairs through spontaneous order arising from individual decisions guided by circumstances is the despair of the Anointed, as it keeps getting in the way of their Visions.
So whatever individuals choose to do is moral and should be legal? What about infanticide? It was part of the "spontaneous order" of ancient societies. Was it moral? Should it have been legal?