> I do believe there are some protections against companies firing everyone when they attempt to collectively bargain. Which is nice I guess. But not a requirement to form an effective union.

Part of the theory of a union is that the workers aren't easily replaceable as a group. Doubly so in that a company that is willing to fire N thousand people and hire replacements will likely have a harder time finding people willing to work for them. That seems sufficient to have enough bargaining clout, even with at-will employment.