If we want to lower the birth rate to a sustainable level isn’t the most effective way to transform people’s lives so that they’re more like those ‘enjoyed’* in the first world ? If so then there could well be a viable long term future for us. The problem is how we get from here (too many people messing up the planet) to there.
*It isn’t clear to me whether the main driver for low birth rates in the first world is that we no longer need a load of kids to ensure that there are plenty of people to look or after us when we’re old (a comfort, and so A Good Thing) or that enjoying a good standard of living depends on bringing in two salaries, so neither adult in a couple can easily afford to spend their twenties having children and much of their thirties bringing them up (a stress, and so perhaps A Bad Thing). And there are always exceptions of course (Jacob Rees-Mogg, Jamie Oliver, my mate Graham).