Peri, I find it all kinds of difficult to understand, too. I had clothes from the 80s in my closet when I joined YLF. I hadn't worn them for many years -- but that was because they didn't fit any more. I kept them anyway. That sort of tells you something.
I also agree there is a happy medium. That was just nutty. Better to donate while they still had life in them -- before they became moth-eaten and so out of style that they could only be used as rags.
My idea now is sort of like my eating. 80% "clean" and 20% "dirty." In other words, 80% of the things I buy I hope to wear, at minimum, for two or three years. (With a smallish wardrobe, that's quite a lot of wear -- but the fact that I have four to six seasons means it is less than it would be for someone in a more temperate climate.)
Meanwhile, 20% of the wardrobe will be thrifted, fast fashion, or just "mad" items that I may or may not "marry." If, when the season ends, I want to pass that 20% on, no problem -- off to the consignment, a friend, YLF style exchange, whatever. The rest I will hope to use for anywhere from 3 seasons to 10, depending on the item.
Thinking like this helps me purchase more mindfully. If I want to try a super trendy item, I need to ask myself it this is "the" trend of the season for me. Is this the one that will be a landscape changer? If not, and I don't think the trend has any legs, maybe I can let it go.