If workers in general are being underpaid, I think they are justified. But then you have other industries where they have a good grip on it and then take things a bit too far.
But once an industry is generally pretty safe and relatively fairly structured, unions begin to lack a certain sense of purpose. Union leadership is elected and "keeping us safe and fairly paid" won't get you re-elected. Promising to extract better compensation for your members (even beyond just 'safe' and 'fair') will get you votes, so that is naturally what happens.
There's just no objective way to tell a union that they have accomplished their goals and aren't really needed until/unless there is a threat to those goals, but nor is there a way to tell a company that they are operating unfairly and their workers need a union...