I think people would, rationally, support unions if their objectives were aligned with the needs of society as well as the wants of the workforce. The size of the labour force required to do a particular task will vary (usually reducing over time), and the necessary skill level of that labour force will also vary (usually reducing over time). Fighting against that trend does not maximise our productivity, and the unions have a long history of opposing and slowing down innovations, automation and technological improvements.
It's difficult to not roll your eyes every time Bob Crow comes on the TV crying about how the latest round of changes are going to decrease safety/ruin the service/etc. It's such a Punch and Judy show people are utterly bored of it.