It depends on your definition. If its whether mankind can feed itself into the future and science can stay ahead of requirement, then you are right. If its whether we can share the planet with the other creatures without leaving them lacking water and environment, I think we are close already. Whilst I sympathise with your overall position, I have to disagree on the amount of useable land this accounts for.
And our taxation based on a steady stream of young people joining the work force, paying tax, but claiming back little. In the UK taxation has been kept artificially low by importing young families from abroad. Immigration of a massive scale.
Indeed, quality of life and survival rates are big definers in number of children born. However you don't need a gross wealth gap to achieve this.
There are two categories of surplus population. One is the group that cannot understand life and so is led to inhuman conclusions and must be controlled by others. The other group is those who control without scruple.