I've been buying R-R tapes on Audiogon and Ebay, at least 30 of them over the last 6 months. They date from the mid l950s to the late l970s. Not one has exhibited the slightest "fading," and only one was defective (no left channel in one direction). I expected that playing these tapes with magnetized heads on cheap tape recorders might have taken its toll, soundwise, but it hasn't happened so far. They didn't all sound great, but they didn't when they were new either. The best of them coninue to set the gold standard for hi-rez audio, IMO.
BTW, the tapes already in my collection before the recent spree all sound exactly as good (or as bad) as they did when I first bought them.