This might be a stretch, but here goes. There are significant similarities between unions (perhaps not all or even most) and ponzi schemes.
Getting in early is key. Early investors in a ponzi scheme often do quite well before the gig is up. Similarly, early participants in unions often will cash in with higher wages, health care expenses paid in large part by the younger healthier union members, and pensions. Before the union ultimately destroys the source of the money, some more realistic negotiations may start to take place. Sometimes these include sacrificing new employees with two tiered compensation packages, so the new guys continue to fund the original union/ponzi members.
The unions destruction of the money source can take decades. The more clever and powerful unions then set their sights on a deeper reservoir of cash, taxpayers. But, as we will eventually find out, bailouts and hidden obligations to municipal employees etc... will bankrupt communities and individuals alike. There is no such thing as a free lunch.