I have a few old merino base layers, which have served me well for a few years, and been through the wash at too many times to count. They're much less warm once I get them wet than they used to be, less so than a new merino base layer by the same people, and the one that's the most far gone is my favorite color, so the one I've worn (and washed) most often. All of them are still better than cotton, but they've lost their magic, and they're no good for getting sweaty on a cold day and then being outside for a while to come.

Are the fibers collapsing, is the lanolin is gone from the wash, or is something else going on? Can I bring these back to life, or should I donate them to the cat?