What is Wallerstein's World Systems theory?

We can traditionally consider three core models which theorise how the development gap has come to exist. These are: Dependency Theory, Rostow's Modernisation Theory, and Wallerstein's World Systems Theory. Wallerstein's World Systems theory offers a Marxist critique of Modernisation Theory, which itself argues that capitalism is the driving force behind international development. Wallerstein's response to this is Marxist because it models the world based on two groups: the bourgeoisie, who populate the global 'core', and the proletariat, who populate the global 'peripheries'. The global core are countries which have used their power in the free market to explore, exploit, and export the raw materials and labour of the global peripheries. This is known as 'the three Es'. The core then valorise these materials and this labour, meaning they trade them and receive the financial profits, rather than the countries which the materials and labour came from. An example of this could be the IMF and World Bank's Structural Adjustment Plans, which encouraged unsustainable trade from African nations, such as Senegal's trade of ground nuts, in the 1970s and 1980s. SAPs caused massive debt traps for countries which participated - in Senegal, a debt of US$4.5 billion had been amassed by 2014. This perpetuates and maintains the development gap between the bourgeois and the proletariat, which is contradicts capitalist arguments that suggest that free trade has the power to accelerate and grow peripheral economies.
There are a number of criticisms that can be leveled at Wallerstein's World Systems theory. Arguably, since it was theorised by Western scholars, it is effected by their Western ethnocentricity and therefore simplifies global macro-economics in a way that ignores the sophistication of intra-periphery markets, such as intra-continental trade across Africa, which has historically been considered as a globally peripheral continent, as facilitated by the African Economic Community. Secondly, it can be said that discussing international developments in purely economic terms is also an oversimplification. Other development models, such as Frank's Dependency Theory, considers the power of cultural capital too. Furthermore, it can seem somewhat conspiratorial to suggest that the bourgeois countries work as an organised global 'core' to exploit poorer countries, since in these countries actually pose financial rivalries to one another on the macroeconomic stage.