> Only if the economic system is a giant ponzi scheme that depends on continuous, infinite growth.
Just because growth is good for a system doesn't make it a Ponzi scheme (or some other type of scam). Population decrease _obviously_ harms economies because it leads to fewer young people willing to take on entry-level jobs and more elderly people who aren't working anymore. That's not to say it can't be managed - economic growth is also driven by technological advancements which aren't directly dependent on population growth. But a decline in population would inarguably have _some_ kind of negative impact which would need to be handled correctly.
> liberal democracies and capitalist markets at-scale are incapable of organizing production in a way that doesn't destroy the earth
That's an incredibly bold claim with absolutely no evidence supporting it. And you seem to imply that there exists some other system distinct from whatever you call "capitalism" which can organize production in a less harmful way - again with no evidence.
In general, recent growth of rich "capitalist" countries has significantly outstripped their energy use (an imperfect metric of environmental harm, I know, but it's what I could find) https://data.worldbank.org/indicator/EG.GDP.PUSE.KO.PP?locat....
> It's funny, there was a post recently about "unsolved problems in economics."
IIRC that post was only kind of accurate at best. Popular culture's understanding of economics is hilariously bad.
> Because capitalism is incapable of solving the problem, and the economists know this, so they don't bother talking about it. They pretend it's an issue of state, when the profit mechanism itself is the cause for most externalities.
From where are you pulling the notion that "capitalism" (for some definition of that word) is incompatible with managing externalities? Or that economists somehow just ignore the existence of externalities?
This kind of attitude is distressingly common online and to me it seems no different from the mentality of anti-vaxxers. Just because a field of science is imperfect and distorted to some extent by government/corporate agendas doesn't give you carte blanche to throw out every bit of research ever done and substitute your own reality.