Perfectly sound line of reasoning. The obvious counter is to point out that many of the world's biggest economies require young productive folk paying taxes to support the elderly. That's a problem for developed economies with falling birth rates and low immigration. And the counter to that counter is to point at the economic model that bakes in the requirement for growth.
Remaining childfree avoids a lot of exhausting and messy complications. Just as remaining celibate avoids emotional entanglements and STDs, at the cost of loneliness. Historically, virtue has attached to celibacy in many religious traditions. Maybe a sufficiently strong and pervasive environmentalist worldview could impute virtue to the childfree. It is possible to change popular ethics in a decade or so: cf LGBTQ rights or attitudes to drink driving.
Remaining childfree avoids a lot of exhausting and messy complications. Just as remaining celibate avoids emotional entanglements and STDs, at the cost of loneliness. Historically, virtue has attached to celibacy in many religious traditions. Maybe a sufficiently strong and pervasive environmentalist worldview could impute virtue to the childfree. It is possible to change popular ethics in a decade or so: cf LGBTQ rights or attitudes to drink driving.