There are plenty of things that can be mended, but time and fabric aren't really the best of friends. No matter how many times you patch up that pair of jeans, or sew up the pockets coming apart on that jacket, you just have to learn to let go. Time is the ultimate test of a quality garment, and unfortunately, no matter the sentimental value, there's a lot of clothing that just isn't up to snuff.
You've heard "buy less but buy better" before, and after months, or even years, of wear, the difference between shoddy construction and well-made pieces become self-evident. It's a no-brainer that if a particular piece of clothing looks more suited for a homeless person, it's time for it to get replaced.