There’s a quiet part I am going to say out loud. On a global scale, population decline isn’t an issue. The problem is /where/ the population decline is happening. Most of the global birth rate is made up by countries with low GINI indexes, low education, and high religiosity. Most of the decline is in high GINI index, highly educated, secular countries.
This population shift has a seriously high risk of causing the global failure of egalitarian democracies, and pushing humanity back into a highly repressive and dangerous state.