I don’t think the women asking on the style and beauty boards if x,y or z is fashionable are childish. I think it’s often coming from a place of uncertainty of their place in the world. Especially given the demographic of the site has a lot of users who have had children, been temporarily out of the workplace or drifted from friends who don’t have children yet etc and are worried that they’ve lost themselves. Women get the double standard of being judged for their looks whilst simultaneously being told that fashion etc is silly or childish.
It’s also interesting that it’s arguably the creative art most associated with women and thus comes in for the most criticism as not sensible or frivolous.
If somebody was quite literally throwing away their white trainers etc because vogue said so and wearing an outfit they hated/felt terrible in because instagram told them too then I think you’d have a point but I imagine (hope) people this extreme are relatively rare outside of Ab Fab.
I also think that ‘fashion’ is more diverse now in terms of looks/trends (long way to g in representation on and off the catwalk though!). There’s normcore (which ironically a lot of the ‘I don’t do fashion’ posters likely fall in to), regency revival, dark academia, e-girls, ballet core, cottagecore, maximalism, twee, indie sleaze plus the perennial goth/grunge/hippy/preppy etc. You seem adamant that there’s a single look but I just don’t see that in contemporary fashion. There are common master trends (things like the cut of clothes etc) but how they’re interpreted differs wildly.
Here are some examples of women who are famously have an interest in fashion/style or at least utilise it as a tool, their clothes will be direct from the catwalk/latest collections or have been picked by a stylist who is at the heart of the fashion industry - Jodie Turner Smith, Tilda Swinton, Lily Collins, Michele Obama, Jenna Ortega, Carey Mulligan, Caitriona Balfe. Surely you can appreciate that all these looks are ‘fashionable’ but they are by no means interchangeable and that each women has maintained their self expression?
The fun of fashion (if that’s not too childish thing to say) is pulling it together to find something that suits you and that you love. Fashion is a way of injecting creativity into my life. I have a non-creative job but I can be flexible in what I wear - yesterday it was wide leg rose print trousers with a slouchy jumper and platform snake-print boots, the day before it was a vintage 1970s check dress with a cropped cardigan and cowboy boots, tomorrow probably a t-shirt dress with a bright sheer shirt dress over the top. I am under no illusions that how I put these outfits together is not removed from current fashion trends. The dress may be 50 years old but there’s currently a western revival and the check fired off the cowboy boot association in my brain. I’m not opting out of fashion by wearing vintage/shopping in second hand shops - I’m just accessing it in a different way.