Is it "at will" for the employee when he knows that walking out means losing his house and starving his family? Coercion and duress come in more than one form. Been there, done that.
There's a reason unions got started, in the first place. If I read you correctly, though, you don't believe that employers with unlimited power would ever abuse their employees. You believe the Muckrakers were just that, and that they never had a bona fide cause. The workhouses, coal mines and slaughter pens never existed; that chapter in our history never happened.
Unions in the past served as a check on the robber barons. However, they were necessary mainly because the robber barons had government on their side (or, more accurately, in their pocket); and could abuse their workers with impugnity. Unions were formed to oppose them, and force changes resulting in better worker safety and and equitable treatment.
Modern unions no longer serve that function for the most part. They're generally not all that concerned with safety and equitability; but in lining the pockets of their own officials. They have a stranglehold on several major industries, and in many states, it's pretty much impossible to get a job in any of those industries without belonging to a union and paying a massive amount in dues every year, with little real benefit from it.
They are, in large part, the reason that American manufacturning hasn't been able to keep up with a lot of the rest of the word in flexibility and cost control. It's also reduced upward mobility and incentive to excel in industries where they have a lock, since promotion and recognition is far more often based on seniority (and good-ol'-boy politics) than it is on ability and effort. In some industries, it's nearly impossible to get rid of low-performing or problematic workers because of the miles of red tape created by the unions. (This is particularly bad in education, one of the worst and most poorly run unions in the US). Likewise, individuals have little, if any, say in how their job is run; and are unable to directly influence it.
As for "starving", "coercion" and "duress"... try finding ways to feed your family or going to work when your union decides that they're going on strike because the union leaders want to squeeze their employers for an extra few benefits for their old-guard (usually at the expense of their lower-ranked members); and you're not allowed to work without finding a new job in an entirely different industry.
They're a good idea, but their current form has long since outlived it's usefulness.
