Unions are basically useless in a healthy market economy because then companies have to compete for customers and employees instead of having a monopoly, which causes them to have thin margins and therefore leave nothing on the table for collective bargaining to extract that wasn't already being extracted through competitive pressure.

Meanwhile unions in a consolidated market have the perverse incentive to sustain the monopoly because then the union is extracting a portion of the monopoly rents the corporation is squeezing out of consumers at the expense of the 99% of workers who don't work for that specific company. Which is why consolidated markets need not unions but antitrust enforcement.

Current US law forces companies to negotiate with a union if it's employees vote for it. That seems like the opposite of a healthy market; it is a market in severe regulatory capture.

A healthy market would allow voluntary decisions by both parties. It would allow management to choose whether they want to negotiate with a collective broker, and it would allow workers to choose whether they want to find employment congruent with their preferences to either self negotiate or hire a third party.