One important function of unions (which is often overlooked) is training their members to be future proof, so that when their job is taken over by the machines

One important function of unions (which is often overlooked) is training their members to be future proof, so that when their job is taken over by the machines they can go do something else, straight away. YC does this sort of duty pretty well. Of course, the economic role you talk about is also important, and YC probably aren't doing that part - but then, they don't actually bill themselves as a union :)