If unions really manage to drive up wages, why would any company want to hire anyone in a union? (Especially for gigs where you can hire anyone worldwide.)
Also in countries with well established unions like Germany, the employers expect the union to be equally responsible for some [productivity guarantees in return. It's less of a one sided deal. I believe in the recent recession, many union workers there agreed to a lesser pay in return for the employer to retain the whole of them employed.
Or is there some legal protection that says you can't fire people who refuse to work (ie strike) if and only if they are in a union?
Of course, the boss would have to have a reputation / commitment to acting that way, so that people don't try.
https://en.wikipedia.org/wiki/United_States_labor_law#Right_...
Make it clear that this will happen, and if your commitment is credible, you'll never see a strike.
The "blue flu" is a traditional way get around that type of threat. A particularly effective tactic is "work to rule", which is a de facto strike specifically designed to get around threats from management; it's harder to justify firing someone for following all safety/regulatory requirements.
If an industry is being particularly problematic, this is where it pays to be on good diplomatic terms with other unions, because the "nuclear option" for a network of unions is the General Strike. If management has any sanity at all, they will choose to negotiate well before such tactics become necessary.
Strength in numbers is usually a winning strategy, and the most effective counter is to convince people that they shouldn't fight back.
Shutting down is mutually assured destruction. But like mad in the cold war, it might work to keep the demands at bay. Iff you can make your commitment stick, and people have nowhere else to go.