Question: So is it money v population that determines the overall trend?
The one consistent trend within the correlations that is part of the decline and fall of all empires, nations, and city states, is that as that society becomes more affluent, the birth rate ALWAYS declines. This is why Social Security and Pension funds tend to fail. They rely on increasing population to pay for the last. This is like creating a weather model under the assumption that because today it is bright and sunny, it will remain that way for the rest of time.
The debt burden then rises as government cannot meet its promises, as was the case in Rome, that is reflected back as raising taxes PER CA PITA because the population shrinks and this ultimately leads to civil unrest and the collapse of the system. Even Augustus (27BC-14AD) introduced family laws to compel men to get married and have children. He saw the immigrants were growing faster than Romans. At least Rome bestowed citizenship on everyone so there were taxes. We try to prevent immigrants, build walls, prevent their citizenship, so they become exempt from income taxes. This is one reason to abandon income tax and move to a consumption tax for then everyone pays regardless of their citizen status.
Thus, Social Security has now turned negative because there are fewer new workers to support the previous. The system is doomed in less time than Rome because they granted citizenship to everyone whereas we do not further curtailing tax revenue growth PER CA PITA. NO degree of shifting money to a gold standard will prevent the cycle from unfolding and in fact that would merely accelerate the demise for the gold would quickly vanish and then what? The system is fatally flawed.