Underpopulation is a myth, the population is actually continuing to grow and even recently reached 8 billion

'Under population' is an idea that only exists because the ponzi scheme of social security is nearing its end. No matter how many people you have, no matter how much they work, no matter how much they pay into the system, the following generation needs to double that to cover it.
Hence 'under population' with 8 billion people..