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.
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.
Most countries in western europe have a concept of day-fines. It's averaged income minus essential expenses multiplied by some amount of time.
Fictional example: speeding 20km/h above the limit gets 30 day-fines. So they'll calculate how much you'd earn in 30 days, subtract essentials and that's your fine.