Neither is 'behind the times' or progressive. It's really six and two threes with both having pros and cons. Some schools handle it badly, both schools with uniforms and those without and different parents and children have different preferences.
The school will have to enforce some kind of dress code either way, and even with the most relaxed rules, you'll get those pushing the rule whether it's the parents or older child. Some schools are too strict on their dress codes and cause unneeded disruption to learning that they claim to be trying to avoid with their uniform and some are too lax causing confusion and conflict between those who follow the rules and those who blatantly don't and nothing is done. Schools have to find that balance and that can be difficult, but I think consistency is far more important than whether or not they have a uniform.
I went to non-uniform schools and so did my oldest child. My younger three all attended a secondary with a uniform. Two of them found not having to think about their clothes relaxing, and for my oldest, not having a uniform wasn't part of his decision making.
I don't buy that not having a uniform means children are more free to express themselves. Aesthetics is not the same as individual expression and for much of education, parents have significant control of that. Dress codes are as much for parents as they are for the children. And really, if everyone is sitting around in hoodies, what are they expressing?
I also don't entirely buy the idea that uniforms are cheaper or do much to save kids from bullying on appearance. All my uniform kids have had issues with being targeted for their looks. I also don't think wearing a uniform is really needed to be able to dress for the workplace, plenty of kids like my son go on to work in uniform jobs just fine. It is easier for schools to get companies to donate uniform pieces in bulk and for the public in a city with many schools, it is easier to report to a school when there is an issue with students if they're in a uniform that identifies the school. Even at my non-uniform schools, there were PE uniforms in part for this purpose because some of the PE happened outside of school.