Unions were formed as a reaction to poor working conditions in factories and payment of low wages. Have unions outlived their usefulness?
The answer to this depends largely on one's political point of view.
In some ways, unions have outlived their usefulness. The sorts of conditions that gave rise to unions in the 1800s are largely absent from American workplaces today. Government regulations have stopped the worst of the abuses and so there is much less in the way of need for unions today.
On the other hand, one can argue that unions are necessary to combat the decreasing pay that workers get compared to management. As American income inequality increases, liberals tend to argue that unions are still needed as a way to reduce that inequality.
The ultimate answer, then, is a subjective one, not something that can be objectively determined.