It's very simple. The United States, in the second half of the Nineteenth Century, quickly became the world's largest economy as the Industrial Revolution expanded at full tilt. The new industrial reality was that workers were not protected from severe bodily harm or death and were exploited in various ways, from low wages to being paid in credits at a monopoly "company store". This was all generally bad, and unions arose to combat it. That was good. In fact, I'll go even farther and say that American unions in particular chose the best possible path, steering away from the socialism and syndicalism of European unions in favor of collective bargaining.

America's growth continued, and after World War II, with Europe and much of Asia in ruins, communism keeping large parts of the globe outside the free world market and America the only place relatively untouched by war, living standards went up to an artificially high level for a lot of people in the US. And here's where the problems started: the unions got arrogant and greedy. By the early 1960s, there was hardly a factory in the US that wanted to run the risk of allowing a strike, and companies caved to ever fatter contracts for union employees, who came to see them as "fair" and expected bigger and better things with each new contract negotiation.

The problem is that all this was happening when America's share of the world market peaked, and it's been in decline since then. Unless there is a Third World War and we are somehow not involved directly, America will NEVER account for the share of the world's manufactured goods, agriculture or really anything else, that it did in the 1950s and 1960s. It will NEVER come back. It was created by a set of circumstances that were almost unique in history.

That's not to say that America won't remain a large and powerful country with a good economy. It's just saying that we shouldn't be measuring things by the 1950s or 1960s economically. But the problem is that unions keep doing exactly that, and each time a business owner tries to say, "Well, I can build factories in China, or Mexico, or India...or even just in South Carolina or Texas...give me a reason to build something in your union-friendly state..." they can't provide a reason.

There is no effective union answer to that business owner's question. They start talking about the benefits of having union workers (there are none - union workers work less and demand more, and they cause problems every 5 years or so when their contract comes up for renegotiation), they sing "Solidarity forever" like they're in a cult and then, like the 18,000 union workers of Hostess, they find themselves out of jobs because labor can migrate.

In fact, there are only a few ways to solve this problem satisfactorily from the standpoint of the union workers: (1) erect trade barriers to make it impossible to ship jobs overseas or (2) unionize all labor. Neither will ever happen. The first "solution" will never happen because tariffs will lead other countries to react in kind (I'm assuming we drop out of the WTO because otherwise we couldn't put up trade barriers in the first place) and trade wars destroy economies. The second "solution" is utopian beyond recognition. It was the goal of communism. Realizing they couldn't compete, communist nations decided that the only way they could ensure their survival was to make sure that ALL countries became communist. However, it only takes one country to say, "Screw this" for the system to fail, because that country will immediately make more money as a result of having a natural (free market) system that treats labor as a commodity.

Your historical argument regarding the benefits of unions is also stupid. Let's take that "Life of Brian" analogy of yours a bit farther. Asking "What have the unions ever done for us?" is like if the characters in "Life of Brian" asked what goods the Romans had done AFTER Titus had sacked the Temple and destroyed Jerusalem. Sure, they brought aqueducts, roads, and all those other good things, but then they butchered the people and drove out those who survived. Unions did a lot of good historically and no one doubts that, but they also did a great deal to destroy American industry.