Agree and disagree.
‘Non-sustainability’ is an ideological construct. It floats the narrative that societies must learn to adapt to ever greater restrictions on standards of living. Veblen demonstrated profits can be created through the destruction of production. Recently, CP wrote on how, due to failing differential rates of return globally, corporations will seek to impose differential advantages locally, on national populations via more totalitarian governance.
I predict, with the fuller retraction of the American globalist project, an odd future looms ahead: ever increased local production combined with greater levels of austerity among the working classes. Already that is set in play with the hollowing out of the middle classes throughout North America. However, return to local production threatens to rejuvenate the middle class. That threat will be eradicated as governments impose restrictions blaming resource shortages, climate change, etc. It is all strictly ideological and serves of course the status quo.
However, I agree with the sense at least that the natural order seems to prefer balance, which is what you apply in the notion that decreased population sizes are required to balance strain on the planet. But I disagree with your premise simply because fertility declines have nothing to do with questions of ‘sustainability’. People have stopped reproducing while resources are abundant (and, given the science of what makes a thing a resource, can only remain abundant) and living standards are the highest in known history.