I don't think tensile strength is the only factor here, or perhaps I should say initial tensile strength. Nylon and polyester will continue to elongate as long they are under tension. I think you will find that a pre-stretched string no longer has the same tensile strength as the virgin string. The molecular bonds between the polymer strands shear and slip, eventually acting like the pizza cheese. Steel is different(suspension bridge analogy). Once it settles in, it stays there. It's molecular structure is uniform. Yes, its tension can change, even increase, with temperature changes, but it's tensile strength does not. Just ask any guitarist about the difference between tuning a nylon string guitar vs. steel strings.