This is the key bit: "This paper argues that the decline in mobility, dynamism, and entrepreneurship is a result of declining labor demand since 2000. When it

This is the key bit: "This paper argues that the decline in mobility, dynamism, and entrepreneurship is a result of declining labor demand since 2000. When it is hard to find another job, employed workers stay at the jobs they have, impairing their ascent up the job ladder and the accompanying wage growth over careers that historically led to the middle class. Declining entrepreneurship can also be explained by workers’ reluctance to leave large, stable incumbents to start their own firm or to work at a start-up when they cannot be assured that they will have a more stable job to return to. Thus, we find that the concentration of employment in old firms and in large firms mirrors the timing of declining labor mobility due to declining demand."