The rising tide on which bdsm erotica will float?

Another stage in the book cycle. A lo-res pic off my aged mobile phone, taken in my local charity shop (or thrift shop, if you use American terminology). It's Fifty Shades Freed.

So now the third book in the trilogy has completed the cycle from niche to mainstream phenomenon, and into charity shops, maybe the prediction that it's the rising tide on which other bdsm erotica will float is coming true?