Ok, the only way that I'm aware of for humans to live sustainably is for each person to live on the land that produces the food they eat. At any point where you take away a person's ability to grow their own food on the actual land where they actually live, you end up with a temporary over-abundance of food, which induces a corresponding population boom (check a population chart and notice the ridiculous spike that begins around 1890). This population spike has an upper limit. At some point the food supply will not be able to meet demand. Even if we all switch to a "super-efficient" all algae diet, we will still hit a maximum sustainable population number at some point.

And then we'll all play a high-stakes game of musical-chairs.