Under 1 billion is unrealistic but some contraction will happen. The main factor dictating how many children people will have is infant mortality of the previous one or two generations as well as the existence of pension systems.
…which is the reason why developed countries have birth rates below replacement level and with increased wealth elsewhere it’s also going to happen there, which would mean contraction everywhere. I don’t expect that to keep up forever, however, states will get their shit together and set incentive structures (in particular making having kids affordable) long before we’re contracting to one billion.