Ancient Rome was characterized by a low fertility rate. This low fertility rate allowed the Romans to accumulate some measure of per-capita wealth, it gave power to artisans and small farmers, it made it possible to fight political corruption, it allowed a significant fraction of the population to do something more than trying to grow enough food to avoid starving today. This meant that wages were higher than subsistence, in other words, there was a “shortage” of workers who had no alternative but to work for sub-poverty wages, which was a dreadful inconvenience to the rich. The rich tried encouraging the Roman people to have “enough” children to drive down wages, but the Roman people stubbornly refused. The rich instituted slavery, but while slavery will lower wage costs, it takes a lot of effort to maintain people in chains, the injustice is highly visible, and while slaves may be mistreated they are not really disposable like landless peasants.
So the rich imported increasing numbers of barbarians, to eliminate a “shortage” of low-wage workers. By increasing the population the Roman rich drove wages down, to the point that ultimately they didn’t need the institution of slavery any more. The brute yoke of supply and demand when there are a thousand starving workers for every new job is a more efficient taskmaster than any Simon Legree. The profits of the rich were maximized, but the overall disposable wealth of the society fell. Rome became weaker, and less able to fend off foreign invasions or internal rebellions, yet ever more dependent on imports of food from conquered provinces to feed a growing population. The empire became corrupt and decayed and fragmented. Some of the rich looted everything not nailed down, fled to Constantinople, purged their army of all non-ethnic Romans (so much for ‘diversity’ as anything other than a temporary expedient to lower labor costs!), and lived in luxury on their stolen wealth for 1000 years. Others became feudal lords living as absolute monarchs over their private estates. And a few of the rich were killed in the collapse, often by the same barbarians they had earlier imported as a source of cheap labor, like Emperor Valens. There is some justice in your world.
Medieval Europe was the libertarian paradise that your neoliberal economists so covet. All land was private. There were no meddling government rules. The relationship between employer and employed was one of free and open negotiation. Landowners were free to offer wages or demand rents that provided only the meanest existence. Workers could accept these offers, or starve, as there was no unused land, everything was owned by a handful of wealthy families, and with a vast excess of workers if they didn’t accept subsistence wages someone else would. But even more, the wealthy could demand total control over the workers’ private lives as well, because the threat of being fired was tantamount to a death sentence.
Medieval Europe was essentially a typical third-world society, like modern Bangladesh without foreign aid. Feudalism is capitalism without capital. While it might seem that low wages would be a spur to economic development – after all, aren’t the neoliberals always telling us that low wages will free up money for investment? – in practice the opposite is true. When low wages are due to a large impoverished population desperate for work, it means that the society as a whole is capital-starved and incapable of making significant society-wide investments. The odd castle or mansion is nothing compared to the resource costs of (for example) universal education, building a comprehensive network of roads and bridges, industrialization, maintaining a large professional army, an ocean-spanning merchant marine, industrial-scale electrical systems, etc.
In accordance with modern parlance I will refer to societies where high maintained fertility rates result in subsistence-level wages as being third-world. It is inaccurate to refer to third-world societies as “developing”, because they never actually develop into anything other than an even bigger mass of poverty. A first-world society is one where moderate fertility rates combined with halfway-sane economic policies has created significant per-capita wealth.
A third-world country can industrialize if first-world countries provide gifts of technology and capital, but this is no panacea. Historically epochs when first-world countries are both available and willing to provide investments are rare; the investments never cause ‘development’ but only feed a population explosion; and sooner or later either the ability or the willingness of the first-world countries to provide inputs of capital and technology will end.
Third-world societies provide a high standard of living for the elite, but they are always vulnerable to external attackers. This is not such a problem if all your neighbors are similarly impoverished, and Medieval Europe muddled along with its aristocrats living like kings, the peasantry living in filth, occasionally nasty local wars and somehow avoiding being exterminated by external forces like militant Islam (mostly because militant Islam was never very strong itself). Until the Black Death came and created modern Western civilization.
A civilization that your elites are busy trying to destroy, so that the good old days of unlimited cheap labor and easy profits can come again. Unless you humans wake up and stop letting the rich breed you like cattle, the elites will succeed, and the misery currently confined to places like India and Pakistan will become the norm for the entire world. It’s up to you, humans.