Buy it for Life

Broadly speaking, consumers should educate themselves about apparel, and then choose to buy apparel that's better made and will last longer. It's not just underwear. Jeans are a favorite example of mine. Most jeans right now are in the 9-10oz range, and have 1-4% spandex woven in so they stretch. 50 years ago, most jeans would have been in the 12-14oz range, no spandex. The would shrink in the wash, so you had to be careful, but would also slowly break in and mold to fit you. Jeans with spandex are more comfortable right off the bat, and can be made comfortable even if they're fairly tight, but as they wear, they're going to start sagging. And since they're a lighter weight material, they aren't going to last as long. The changes are, in large part, driven by the need to ensure that your jeans fit a wider range of body shapes; your fit doesn't need to be as specific when you use elastic. (That starts wading into the deep end of fitting apparel, but the short version is: patterns can be pretty easily graded to fit people that aren't overweight, but once you get past a certain amount of body fat, distribution and shapes start varying widely enough that you simply can't make anything that's close to universal without making it fit like a poncho.)

There was even a brief period of time where Invista had a partnership with a mill that was making denim, and they were doing 60/40, or 50/50 cotton/nylon denims, and they had fantastic wear capabilities. I haven't been able to find anything about that particular material in about 15 years, sadly. (Cotton/Kevlar blends are possible to find; those are used for motorcycle jeans. They're also $60/yard for 30" wide fabric, which is insane.)