I really did not think there was anything controversial when I posted. My own experiences are that in fabrics, natural fabrics are less sun resistant than synthetic materials. In my limited experience with thread, the only decayed thread I have ever replaced was to a coworkers boat top. After about ten years the top fabric was in very serviceable condition, but the ( what looked like) linen thread was decomposing and the seams were opening. I resewed it with nylon and all was good. Not scientific at all, just my observations.
Aaron