Why not? It's appropriate to dictate and even advise on lots of things that are personal/private.
We have laws and social norms to keep people from being horrible to their own children, even behind closed doors, but only after those children are born.
My suggestion is for those bits of advice, backed by the government's monopoly on force, to kick in just a year earlier than that.
Carbon taxes/credits are probably possible. Huge public works (build enough solar panels and energy storage station to meet most of our power needs). Big taxes on carmakers for making high emission vehicles, etc. There are lots of policies which can work, and I think the market will, eventually, also probably get there eventually. But just telling people not to have kids is silly.
I'm not really advocating anything here, just lamenting the seeming (and somewhat assumed) immutability of it all.
(random but I really appreciated these subthreads, definitely some good stuff for me to chew on)