I agree that it’s definitely hard to find a nice pair of plaited reins new. I have gotten the majority of mine gently used for this reason, though they’re not the easiest to find on the used market, either. Last I knew, Hadfield’s makes some that I’m sure are lovely, though definitely pricy. I used to see some from HK Americana as well, but not for awhile. I do have a pair I bought new from Tory 10 years ago that are pretty nice, and are thinner than the usual kind. Any of the cheaper brands other than Tory seem to stay stiff forever rather than break in and become soft like the nicer ones do.
I also agree that often times, rubber wears out prematurely, and I’ve had/seen plenty of the Rainbow Reins melt (though those are very cheap reins, so probably not surprising there). I’ve seen rubber reins dry out and begin to crack over time, or just wear out from use. I don’t buy these often for this reason, as I have ancient leather reins still in use and tend to want to buy whatever lasts the longest. How long have you been using the Nunn Finer reins? I do like the feel of those, but typically don’t buy rubber because I’m afraid of it melting or otherwise not lasting.