Famine is the arbitrator of sustainability. There were twenty great famines in Japan during the Edo period.
With the advent of birth control, famine is not, in principle, necessary. In developed countries, birth rates have plummeted. Birthrates in developing countries are falling but not nearly enough.
Famine (foodwise) would not be a problem if humans were willing to produce excessive food, and the foodstuffs reaching the end of their storage life turned into a storable energy But that would take peopel not working to maximize their own personal 'universe', but have a working for a common good. Alas, anytime there is a group of people, a few will work to screw the others over to benefit themselves, so I'm not hopful for a famine-free regulated envirnment.