My pet theory is based on evolution: A harsh competitive environment forces commercial entities to develop ever more effective means of extracting value from t

My pet theory is based on evolution: A harsh competitive environment forces commercial entities to develop ever more effective means of extracting value from their workforce, while labor fails to adapt in response because its corresponding entities (chiefly labor unions) are not subject to the same harsh competitive pressures. Eventually you have an accumulation of durable local wage minimums which in the aggregate appear as a global decrease.