It is not the poor, sparsely populated areas that are of concern. It is a place like Japan. They have over 120 million people crammed into a place the size of California, and most of the land is uninhabitable and inarable. As creative as they are, they have forced on themselves a lifestyle that is unstustainable. They are not able to produce enough food for their own people, so they overfish waters of other nations and import food that is very expensive. They are not reproducing at a replacement level, so when the check comes due for all the people who will eventually retire, they will be in big trouble. They have insanely long lifespans there.
Our planet can sustain a much larger population than we currently have. You could fit every person on the planet in the state of Texas. Unfortunately, big government has imposed regulations that penalize generating food, income, and shelter for everyone. This planet has resources to be able to sustain more people, but government gets in the way.