They can't get a special deal that pays you what you're worth, doing what you're best at in that industry. Average workers, though, don't get what they deserve, either. Nope, not even most of them. So they will now be against below-average workers, who otherwise could have gained the experience necessary to make a good living in the industry, starting out making what they deserve and then working their way up. Second, the union bosses can't keep their power if the company goes out of business, so they will support this discrimination against below-average workers.
All workers also lose the power to choose the benefits they desire, while they're at it.
You're dealing with a monopoly here, so nobody cares what you want. Of course not all people in a union get paid the same thing. And, while we're at it, you're not just getting ripped off as a worker, either. And yes, the companiy can be hurt even while the workers are. The union acts as an unnecessary middleman, taking away the power of both employee and employer to agree on individual deals which benefit both sides, so the union hurts both sides.