[skew-aberration]

There's always work available if someone is asking for a low enough wage, that should be self-evident. How does this prove that the population at large benefits from more people being willing to work for low wages?

Classical economics would argue that higher population tends to increase productivity (economies of scale and specialization) while lowering wages, increasing land value, and improving returns on capital. Free trade has a very similar effect. You can find this analysis in all standard texts (Smith, Riccardo, etc) and even progressive works such as Progress and Poverty by Henry George.

Different groups are free to have different opinions of this tradeoff. Big landowners benefit immensely while working class renters only suffer.