A) Low birth rate leads to a Population/Population disposable income total(not per capita) will decrease, and thus gross capitalization decreases reducing stock value. Decreasing the benefit of investing in index funds potentially catastrophically. B) Inflation will be/remain high(or above earning increases) for core expenses (housing, food, water, gas, transport)
Sometimes with the argument that B happening will cause A to happen (e.g. more expensive = less attractive for young immigrants, less attractive to have kids)
I agree with some of it (for example disposable income inequality and core inflation issues are indeed causing a cost of living crisis which is discouraging growth), but I advise NOT to go down the rabbit hole of these channels, as they can give you a lot of anxiety.