As music has moved on from the iPod, it has actually become much less important to the tech industry. With streaming and the decline of ownership, there are fewer barriers to switching service, and every device has a choice of music services, both from the platform owner (i.e. Apple/Google/Microsoft/Nokia etc) and as third-party apps. In effect, music has become a commodity.
Compare this to dedicated eBook readers, which still have a feature that smartphones haven't replicated (E Ink). The iPod didn't have exclusive features that tied music (and customers) to the device; the built-in WiFi and cellular options of iPhones facilitated the rise of streaming services and access over ownership. The only iPod-only feature I can think about is storage in the 160 GB iPod Classic; millions of people just don't need an iPod anymore.
For further evidence, look at Apple's upcoming launches: iTunes Radio can be used on iPod touches connected to WiFi, but iPhones (and cellular iPads) can use it all the time.