>I am gobsmacked. Where was the air traffic controllers' union? They could have used industrial action to address the unsafe conditions

Think about the incentives for a few nanoseconds.

If you think a union wouldn't wheel and deal and enter into some handshake agreement to "table this one concern to next year" indefinitely in exchange for something I've got a bridge to sell you.

Adding another entity isn't gonna change the fact that the local controllers will still be the only people who care (because it's likely their ass if something happens).