I don’t think this is actually true. Assume zero GDP growth and zero change in the ratio of income from capital and income from labor. The share price of your stocks does not increase or decrease, but they pay you an annual dividend. The return is still positive even under overall stagnant conditions.
...assuming either a constantly growing economy, or a constant flow of wealth from those without investments to those with them.
I don’t think this is actually true. Assume zero GDP growth and zero change in the ratio of income from capital and income from labor. The share price of your stocks does not increase or decrease, but they pay you an annual dividend. The return is still positive even under overall stagnant conditions.