I don't think your conclusion at the end is true. Most undesirable work is in fact not valuable, in the sense of being paid well or easy to get, because, at least if it's also unskilled, there is a large supply of labor relative to demand. Stuff like janitorial work does not pay well and has no shortage. Similarly with McDonald's: they typically get many more applications than open positions, and not because they're paying $20/hr.