Nigger, we are already overpopulated, most of humanity relies on fertilizers to survive, artificial fertilizers that come from ground and are not renewable. We will run out of most fuel in 100 or so years, like oil, coal, gas and even fucking Uranium. There are only handful of people living in developed nations, the rest is living in shitholes with rising living standards that will soon match the developed nations with many times the population of developed nations. There are more cubic meters of human biomass then the biomass of all wild land animals combined, just let that sink in. Humans are the most overpopulated species on the planet and need severe reduction to their population in order to make them sustainable with their habitat (in this case the whole Earth). The reason why it happened in the first place is because we are not dumb fuck animals that only use renewable resources and once they start running our we starve out to sustainable numbers, we invent shit to change nature to suit us and mine or pollute stuff to buy out temporary materials to keep the population alive. The reason automation by machines is largely hindered is because there are more people then material to make robots, so it is cheaper to pay some chink pajeet in central africa or whoever to do robot job. Even first world nations with super high minimal wages cant afford them.
Go preach underpopulation meme once we hit bellow half a billion global population, until then the only drawback of low birthrates is aging population, which can be fixed by just forcing people to not retire and work for way longer into their years.