The thing is "classic old-fashioned" isn't something that is somehow timeless and is somehow outside of fashion trends. It's right in the name. It was fashionable but now it's old, old-fashioned. You can throw the "classic" in there all you want. The truth is all clothes come from some particular fashion. Some trends tend to change slower than others and that's what people seem to refer to when they say "classic" but it will change.

Wearing jeans at all is a trend. People have been wearing something other than jeans for far longer than they've been wearing them.