> Unions don't magically create leverage out of thin air. They steal it from those outside the union by constraining the the supply of labor.

That sounds like a perspective from one of Amazon's workplace signs. Union leverage is based on two things (off the top of my head, and afaik): significant control of the labor supply, and the political power of many votes and of many people to advocate for candidates. However, people can join the union; they aren't exclusive clubs.

Much of management's power comes from controlling the job supply and the political power of money, and they use that leverage to their advantage. When we live in a happy world where management stops doing that, I'll agree that unions shouldn't either.