Eh, it's a worrying scenario but I think it would be taken care of by intertribal competition. The people of Idiocracy would be easy pickings for a group of raiders that prized intelligence highly.

Really, the only two countries that are combining a knowledge economy with an expanding population are Israel and India, both of which essentially have a tech-worker economy subsidizing and intermarrying with a bronze-age economy. That seems like the likely actual solve, a few ivory tower cities surrounded by amish country. Every now and then an Amish tests into the cities or a city-zen absconds to a simpler life, but the net flow is generally intellectual country types moving into the tech hubs.