Usually, within the framework of a discussion about limited resources, they talk about minerals and especially about energy resources. While everything in the universe cools down, collapses and degrades, humanity, thanks to energy resources, develops and reduces entropy. But there are more primary things that are spoken about less often than we would like - the person himself.
Humanity's capabilities depend on two factors - skills and population, in other words, quality and quantity. All other things being equal, twice the population will produce twice the wealth. The same is true for skills that can be called labor productivity. It may seem that “quantity” (population) is much less relevant than “quality” (labor productivity), but this is not so. A double population would give birth to twice as many geniuses and could send two or three rovers to Mars instead of one, scientific and technological progress would accelerate. Therefore, if you dream of a progressive future, then you should not forget about the population. Perhaps this is what Elon Musk recently had in mind when he stated that the world population is heading for collapse and few people take it seriously.
If you look around and look a little ahead, then there really is no particular problem. But Comrade Musk is known for operating and trying to look into a much more distant future, let's try to follow his example.
Real examples
To begin with, there are already model examples in Japan. It is believed that the country has been undergoing an economic crisis for a long time, which has even received its own name - “the lost decades”. But in fact, the crisis is exclusively demographic, it is not difficult to see if we consider economic growth through labor productivity and the number of workers:
The qualitative factor annually provided 1.5% of GDP growth for both countries, but the decline in employment slowed down Japan by 0.3% - a total growth of 1.2% per year. If employment in Japan increased as in the United States, then its economic growth would be almost twice as much - 2.25% per year, which is already a good figure. But history does not know the subjunctive mood - Japan's GDP has slipped from second to fourth in the world rankings and this is not the end: the birth rate in Japan is deteriorating and a recent study claims that almost half of the population in the 18-34 age group has not had sex. There is an epidemic of virginity in the country.
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Therefore, one growth of labor productivity does not leave (fly away). At the same time, the success of China has shown that it is possible to take "quantity" - its GDP in PPP, as well as investments in R&D exceed those for the United States and Europe.
A glimpse into the second half of the century
In the distant future, trends in population and fertility (the number of children per woman) are extremely inertial, and therefore easily predictable. At the same time, it is most interesting to consider the population of the age group 25-64, which creates the bulk of material wealth, which doubly simplifies the forecast - for example, in the distant 2040, this group will only include those who were born in the period 1976 - 2015 and are already known to statistics, new people in this group will no longer appear:
In a couple of generations, the population in the 25-64 age group will halve, with corresponding implications for the ability of Japanese society. Agree, I would not like such prospects for all mankind, but this is exactly what is going on.
The Japanese scenario repeats itself on a global scale
At the moment, in addition to Japan, the West, Russia and China are passing through the “peak of population”:
A little later, it will affect Latin America and India. Without taking into account Africa, the colonization of Mars will take place already on a declining population.
Due to population problems, global GDP growth will slow down by half, which will alienate our scientific and technical fantasies. At the same time, further population growth will occur in countries with very low labor productivity, which will reduce its effect. Perhaps the situation can be changed by a “strong” artificial intelligence, but we must give birth and raise geniuses to create and improve it ourselves.
How to solve the problem of “peak population” is not clear. Many countries have special programs to support fertility, but they don't work. Developed countries solve the issue of immigration, but, in general, this is a zero-sum game - somewhere, after all, it has disappeared. Since such a distant future is affected, then who knows, maybe cloning people (for example, the Japanese in this strange Japan) will become a natural practice.