Then, I set several relevant questions and have sent them to some people and organizations from whom we can expect positive answers.

(A) Isn't it quite possible that our society will not be sustainable if overpopulation and expansion in consumption would occur when it becomes affluent for all people and we get over the poverty and hunger?
(B) If it is, what is the solution?
(C)What is an adequate world population where all people live an affluent life, setting living in Denmark, or the U.S., or other suitable countries as a standard?
Although it must relate to the matters including natural resources depletion, environmental issues, development including space exploitation, and advance in technology, I think we need to seek the answer.

This is a quite serious problem in the real world where more than 20,000 people are dying of hunger day by day.

I will post those answers in this page if I can get some.

→Ｐｏｓｉｔｉｖｅ　ａｎｓｗｅｒ！

One possible solution is as follows:

(A)(B)By peacefully transitioning to an adequate population with the help of women's empowerment and other things, such as "Reverse child benefit(*1)"we can make our society sustainable even if all people enjoy an affluent life, overcoming the poverty and hunger.

Moreover, overcoming hunger and poverty; and educating poor people will lead to the expansion of consumption (economic growth,) namely, the leveling up of the world economy-- these policies will be effective in protecting and fostering the current and potential human resources as both good labor forces and consumers.

(*1) "Reverse child benefit" is a benefit paid to a woman according to the number of children she has: the smaller the number of children, the more the benefit. This has a population control effect in heavily populated areas. It may sound rather rough, but I think it's a good idea

(C)According to information from Wikipedia (as of June 4, 2021), the world's optimum population is 1.5 to 2 billion people (*2)          

(*2) https://ja.wikipedia.org/wiki/%E9%81%A9%E6%AD%A3%E4%BA%BA%E5%8F%A3

According to a study by Paul R. Ehrlich et al, the following were taken into consideration. The calculation seemed to involve:

Adequate wealth and resources for all people
Basic human rights for all
Protection of cultural diversity
Allowances for intellectual, artistic, and technological creativity
Protection of biological diversity

My supplementary note: The calculation in his paper is based on the total amount of energy that can be consumed divided by the per capita consumption of developed countries (*3). It may be that the introduction of clean energy such as hydrogen has not yet been calculated.

(*3) The calculation was as follows.
The marginal energy consumption is 9TW (Terawatt = Terra is the 12th power of 10). The marginal energy consumption is 9TW (terawatt = terawatt is the 12th power of 10).
Realistically speaking, we need to take into account a 50% error, 6TW, additionally considering a 100% error, 4TW is the upper limit of energy consumption.

The average energy consumption of a developed country in the 1990s was 7.5 KW (kilowatts), but with modern technology, this can be reduced to 3 KW without compromising the standard of living.

6TW, 4.5TW divided by 3KW = total energy consumption per capita in developed countries = 2 billion, 1.5 billion

＝the optimum world population = 1.5 to 2 billion

Reference: Gretchen C. Daily, Anne H. Ehrlich, and Paul R. Ehrlich. Optimum Human Population Size Archived 2017-08-17 at the Wayback Machine.. Population and Environment: A Journal of Interdisciplinary Studies Volume 15, Number 6, July 1994 01994 Human Sciences Press, Inc.