That "no" means they have to be willing to replace a substantial amount of their employees (including those who might not be part of the union but are willing to go along anyway when they see the terms of the negotiation), deal with the bad PR the union can bring to bear (strikes at large companies are major national news), find replacement employees who can look at that PR and still say "I'm willing to work for this company", deal with falling stock prices, deal with the public or partners potentially choosing to not do business with them, and do all that while continuing to fulfill existing contracts and orders that don't care what troubles they're having. That's a pretty massive "All they have to do". The wrong call there can put a company out of business.
If they're still willing to go through all of that, and the cost of doing so is still cheaper than granting the union's requirements, then they'll say "no", and the union's attempt to bargain will fail.
> Unless they're afraid of getting fired.
Yes, that's the risk you take. You have to actually be willing to quit and stand with the union if you want the union to fight on your behalf. You have to be willing to be out of work and not get a paycheck. The situation needs to be serious enough to put that on the line.
Unions are an incredible tool, and I'm in favor of them. That doesn't mean I'm in favor of laws enshrining them into disproportionately more power than they already wield.