If having children were a net cost over an unlimited time window, globally, at any time sufficiently far in the past for the effects to propagate, then humans would already be extinct. We can take the fact that this has not been the case so far as evidence that it isn't the case now, but it's not a necessary inference given that the cost of raising kids is not constant. In some cultures, one's offspring might be considered autonomous adults by the age of fourteen with skills their parents were able to impart in their free time, whereas in the present day it's becoming more and more important for the average adult, in order to be hired as a worker, to be credentialed by institutions which are following their own incentives to capture a greater and greater proportion of the workers' future wealth. Other costs may rise similarly.

Additional money may enter the economy (although it's technically possible that negative externalities render even this illusory as an increase in actual wealth,) without utility increasing; as is the crux of the argument, that wealth could be applied to more valuable causes.