> Wouldn't the union only be composed of the same colleagues who were resigned to 'hushed mutterings in corners' and laughed at the grandparent poster for 'being on some kind of moral high horse'?

The answer is "yes". There is no reason to assume that a union representing employees would be any more motivated by customers' interests than the employers themselves would be. (And there are plenty of actions by real-life unions in other industries suggesting otherwise.)