Fabric softener doesn’t wear them down leading to tears; what it does is build up material on the surface that can eventually make your clothes feel different, can make towels less absorbent, can strip other coatings from your fabrics (like fire-retardants) and can leave deposits in your washing machine that can get moldy. It actually makes most natural fibers, like cotton, more elastic allowing them to hold their shape better and gives them a little bit more resistance to being torn.