> Starbucks, Google, Amazon, these companies will spend millions to fight unions. This is prima facie evidence to me that unions benefit workers.

Technically, that does not follow. What can be inferred is that companies believe that unions will cost the company money, but this does not necessarily mean that the employees will get any of that money. One could imagine a theoretical evil union which takes all the extracted profit from the company and gives it to themselves, and not to the employees.