I always think of unions as less a new force in the equation you described and more of a labor side equivalent of the collective organizing and action that happens on the company/shareholder side to maximize value. Instead of individual workers all trying to maximize their value in isolation with the company trying to collectively maximize value for all shareholders, unions ideally make it more of a one-to-one balance of "workers" and "shareholders".

I agree with your general thought that it could be good for business, but I'd take it a small step further and suggest a more comprehensive way to think about what's good for business as encompassing workers and shareholders equally. A situation that optimizes shareholder value at the expense of worker value is not really economically optimal even if that doesn't necessarily damage the business long term. If you can get much better conditions for workers with only a small drop in shareholder value, for example, that could be considered a net win.