Are you attempting to make a counterargument, or are you hoping people will read your comment and go "yeah, corporations bad, down with corporations!"? Yes, voluntary unions have to fight against the ability of a company to simply fire people; they depend on the willingness of the entire group to stand together and take the risk together, win or lose.

The whole point of a union is that employees have more power collectively than individually. A company can just tell an individual "no". If all or almost all employees get together and say "the situation is so bad that we're willing to quit (and make a lot of noise and bad PR for you) rather than let this go on", then that's harder to ignore. If the company is willing to fire everyone and deal with the bad PR, and that's cheaper than the union's demands, then the union doesn't actually have the power to effect change even as a collective.

I don't see any reason that either the company or the union should be artificially given more power in that scenario.