If ever there was a self-fulfilling prophecy, that's it. There's a first time for everything. And in fact, there's a proven way to make people decide to have fewer kids: make them rich. People in more affluent societies generally choose to have fewer kids, to the point where the fertility rate in some places (Japan, Italy) is below the replacement rate. So it clearly can be done.

Wealth is just a proxy. What lowers the reproduction rate is higher living standards. And having all humans on current industrialized nation living standards with current methods of production certainly would not make things more sustainable.

If you want sustainability you either need fewer humans or clean up the entire supply chain. And the latter might just suffer from jevon's paradox.

I didn't say it would be easy to get from A to B. But it's not impossible.

It's also possible that someone discovers a way to build a cheap commercial fusion power plant tomorrow. But we generally don't rely on these kinds of remote possibilities.

That's just not true, people tend to desire less and less kids, a lot of people deliberately chose to have no kids, something that was unthinkable several decades ago.