The market-driven counter to this is that if companies don't offer terms that employees agree to, they won't be able to get any employees and therefore over tim

The market-driven counter to this is that if companies don't offer terms that employees agree to, they won't be able to get any employees and therefore over time, in the aggregate, they will move towards outcomes similar to what would be achieved in a mutual consent arrangement, without the need for unions. Of course if that was correct we likely would not have seen the rise of unions in the first place because things would simply have adjusted over the many decades if industrial revolution leading up to wide scale establishment of labor unions. When it comes down to being able to survive, people will accept the terms they're given. They don't have the luxury of waiting for a "market correction" on labor rates and benefits. Open markets can find efficient solutions to complex problems. Unfortunately, much like waiting for evolution to find a solution for a problem, it doesn't always happen at a time scale suitable for human lifetimes.