First off, when "overpopulation" is used it implies that the population of people in the economy is more than what can be supported by the economy in terms of providing full employment and a reasonable standard of living (or more economically, the aggregate demand will be greater than the aggregate supply).
The worst result of this sort of overpopulation is that there is no money in the country's treasury to provide basic services or to further invest in the economy, resulting in sustained inability to recover. For perspective, it would be as though the Great Depression or the 2008 Market Crash occurred without any method of rebuilding.
In theory, further exploiting natural resources could be an option to escape, but assuming global overpopulation, this wouldn’t be possible. This leaves only a few additional options: extreme (not-for-profit) innovation, global die-off until the population found an equilibrium, or perhaps (though less likely) people would develop a complexly new economic system built around sustainability. The last one I say, as all new economic growth under the current system is based off the exploitation of pre-existing natural resources.
Perhaps, this will never happen. Yet right now, it is almost a certainty. In the past 40 years, the Earth has lost 1/3 of its arable land,[1] and our population increase is not slowing down in appreciable way. Even if the population growth stopped, most of the world aspires to live a lifestyle similar to the one in the US. If that were to be achieved, we would need 5 Earths worth of resources just to be self-sustaining.
The reality is grim. Unless we change our ways, we are overpopulated now. The only reason that it isn’t a problem is that we still have resources to exploit. As a definite, we will run out of oil, the largest of all exploited resources, which we need for more than just our cars. Imagine how many items are plastic (a primarily oil based material). Even if we are all-electric in our energy production, 50 percent of all the plastic we use is used only once before being thrown away (or about 185 pounds); if this waste were to be bio-plastic, that only leads to faster arable land degradation.
The Food and Agricultural Organization of the UN had a report saying:
While in 1990 forests made up 31.6 percent of the word's land areas, or some 4 128 million hectares, this has changed to 30.6 > percent in 2015, or some 3 999 million hectares, according to FRA.
Meanwhile, the net annual rate of forest loss has slowed from 0.18 percent in the early 1990s to 0.08 percent during the
period 2010-2015.
This is still bad. Any rate of deforestation is unsustainable over the long run.
Face it, we are overpopulated. What will you do about it? Comment below.
Footnotes
[1] https://www.theguardian.com/environment/2015/dec/02/arable-land-soil-food-security-shortage