Personally, with so many people now having gone through several iterations of iPhones with many using their older versions as back-ups, as music players, etc, I just don't see a new generation of the iPod Touch on the horizon.
Thats what I'm doing with my iPhone 7 Plus. I use an Android device now, and though I got most of my music over to my Pixel 2, Double Twist just isnt the same as iTunes. I really wish they would bring back the iPod Classic, with flash storage and cheaper pricing, but thats never going to happen.