Have unions lost their true purpose?

I always believed unions were to protect workers from corporate and governmental giants. Be that by attaining fair wages, receiving benefits, enforcing safety decress, etc. But look at GM. Many workers there are receiving more than a "fair" wage and other "fair" benefits that is bankrupting the business. Is their always a trade-off? Governments and corporations attempting to keep the rich rich and the poor poor and unions attempting to make the poor rich but at the expense of productivity and competitiveness?