The company can always say no forever. It's just that doing so normally causes major disruptions in operation and so isn't really an option at companies where a

The company can always say no forever. It's just that doing so normally causes major disruptions in operation and so isn't really an option at companies where a critical chunk of the labor force is unionized. If it's just 10 out of 100,000 employees participating and they can be easily fired and replaced then the union is always going to be toothless. For example I am a union of 1 at my company and can try to negotiate "collectively" but the company can also say no and fire me.