Framing degrowth as a choice (against continued economic growth / business as usual (BAU)), rather than as a planned de-escallation as an alternative to an unplanned catastrophic collapse and/or descent into some post-prosperity feudalistic gilded era in which a vanishingly small fraction of global haves might enjoy something approximating present high-consumption lifestyles but the overwhelming majority do not and never will ... is ignoring the fundamental reality of the degrowth / steady state argument which is that planetary boundaries exist. There are limits to resource availability, as we've known for a long time, as well as to sink capacities --- the ability of the environment to absorb the effluvia of our activities without undergoing a catastrophic shift.

There are also quite probably systemic responses, including (though I'd suggest not limited to) the emergence of global epidemics and pandemics of disease and dysfunction. Kyle Harper's recent book, The Fate of Rome, advances the case that diseases co-evolve with civilisation. This itself becomes an interesting counterpoint to the well-worn argument that science, technology, and progress have relieved humans of the evils of infectious disease. It was progress, population growth, commerce, and urbanisation which created those plagues in the first place. As Jarred Diamond noted in Guns, Germs, and Steel, it was the Old World with its crowded cities and vast commercial networks which introduced plagues to the New (with a very few possible exceptions, and those largely very slow-moving insults).

Though of course, diseases of the tropics were well-known. Those however are overwhelmingly zoonotic and parasite-based rather than trasmitting directly between humans, and reflect the same dynamics (huge populations and intense intercourse) though of other species, typically insects. Diseases of sanitation and hygiene, notably cholera, would be the other principle exception, though that was introduced to Europe largely from the Indian subcontinent.

Again, the point isn't that degrowth is an option to BAU, it's an option to a far worse collapse or oppressive wealth concentration with massive disenfranchisement. The fact that discussing the issue in these terms an on this basis is so readily derailed is itself among the profound challenges in achieving a non-worst-case outcome. I've pointed before to the work of William Ophuls (Ecology and the Politics of Scarcity, Plato's Revenge) and Thomas Homer-Dixon (various) before, I'll recommend them without reservation here again.