Yeah, the whole point is having a legally binding contract that sets wages, hours, and working conditions. Also the "going through your union instead of your manager" is super dumb. It's like saying, "why talk to your lawyer when you can just confess to the police?"
No guarantee as in "theoretically, we could fold up our entire business instead of bothering to negotiate". They won't, of course, but it's not liably false (IANAL) because there's a valid weird hypothetical.