At the end of 20 years a child becomes an adult, who will (on average) be a net producer. It's true that children cost more in work put into raising them than older adults take in work to look after them, yet it is a fact of life that one working human is capable of producing more stuff than just that which they themselves needs or makes use of - otherwise it would be impossible to raise children. This should be obvious, and means that the problem of an increasing birth rate, from this perspective, is at most temporary.

But I didn't say we should have high birth rates, I said that the problems of low birth rates are not due to capitalism.