Sure, if you can invest your excess cash into the stock market and earn more than your interest rate on your debt, it's a net win. The problem is that he is ign

Sure, if you can invest your excess cash into the stock market and earn more than your interest rate on your debt, it's a net win. The problem is that he is ignoring his downside. On a risk-adjusted basis he made a poor decision, which he'll probably discover if the stock market crashes again and he loses his job.