Population growth as a poor proxy for capitalist workforce growth

It is common in papers on economic growth to proxy capitalist workforce growth by the growth of the overall population.

This assumption yields a good approximation in developed countries with stable populations and birthrates. In countries where the capitalist workforce is swollen by people coming out of subsistence farming and moving into paid labour, the assumption could be dreadful. Since most developing countries have exactly such a workforce, the assumption requires urgent revision. It is likely to overstate the estimated effects on economic growth of other forms of accumulation.