Please summarize Sinclair's argument in this paragraph for why longer lifespans will not lead to overpopulation.

Context: A review for Boston Magazine called Sinclair “one of science’s most controversial figures” and said many in the scientific community were skeptical of claims he made about human longevity. University of Alabama biology professor Steven N. Austad said, “David is a good friend, but I do think he’s been guilty of making excessive claims.” Writing in Archives of Gerontology and Geriatrics, Charles Brenner summarized that Lifespan has "become an influential source of misinformation on longevity, featuring counterfactual claims about longevity genes being conserved between yeast and humans, the existence of supposed activators of these genes, and claimed successful age reversal in mice based on partial reprogramming." In a 2019 interview, Sinclair dismissed the idea that longer lifespans could lead to overpopulation. "Population growth will level off within the next couple of decades, and healthier people are having fewer children," Sinclair said. "The global population is already stabilizing, and in many advanced countries going down, so people’s fear that the world will be overpopulated with frail old people is completely wrong."

Answer: Based on this paragraph, Sinclair is not concerned with the risk of overpopulation even if lifespans become longer. His rationale is the healthier people are having fewer children and so the population is going down in many advanced countries.