On average, assuming nothing else changes. In reality, we have the levers of immigration and workforce utilization (e.g. making it easier for ex-felons or non-violent drug convicts to re-enter the workforce) to juice labour inputs. And we have education and R&D to juice productivity. On top of that are random factors like natural resource utilization, falling energy and material intensities of GDP, et cetera.
> That would imply an S&P 500 of about 370 (down ~95%) and interest rates in the 10% range
One cannot so neatly connect demographics to interest rates, let alone the S&P 500. Demographics map to certain consumption sectors (e.g. durables) very well. They map to others (e.g. luxury or entertainment products) quite badly.
To illustrate one of the many confounding variables between demographics and stock prices, consider what a small shift in asset allocations in a single generation from real estate to equities would do to prices.