"[E]very year, somewhere between 30% and 40% of all job-changing participants will cash out from $60 billion to $100 billion in the first year following separation, and about two-thirds of that leakage is ... not driven by a true financial emergency.... When ... small-balance participants change jobs, they face challenges.... Unfortunately, the 'path of least resistance' is to cash out when the easiest choice should be to move their retirement savings forward."