I agree, it's really subtle. I've been surprised by how dated some of my things looks when they're only a few years old.
Skinny jeans, for example. If I look at my pairs going back over the last ten years, the fit and cut have changed a lot, although only subtly from year to year. My early ones were much much longer, slightly flared around the ankle, and cut lower on the waist. More recently, they are skinnier, stop at the ankle bone and have a much higher waistline (even though they're not officially mid or high rise jeans).
Little things like that make things look dated.
It's why I'm never sure I agree with the "spend a lot of money on classic items". Even a plain white shirt can look very dated after a few years, because fit, body length, sleeve length, collar cut and thickness, etc all change subtly from season to season.
There are a few things you could do to get more wear out of things. For example, instead of binning my dated long length jeans, I hacked the ends off them to make them shorter and left them unhemmed as that was quite fashionable at the time. I'll probably bin them when that look passes.
One tip is to search for an item on Pinterest and see how they're currently being styled, for example "black tapered trousers" and have a look at the differences (or similarities) between your pair and the outfits in the picture, and see if there's anything you can do to update.