Off the top of my head, here's how I've experienced portable music players changing:
1. Cassette Player ("Walkman")
2. CD Player
3. MP3-capable CD player, mini-CD player
4. Proprietary medium player (mini-disc, etc.)
5. MP3 player with internal storage, some expandable (Diamond Rio, Creative Nomad, Archos Jukebox, Apple iPod, Sandisk Sansa)
6. Smartphones (using local media collections) & surviving MP3 players in the market
7. Smartphones (using streaming media collections) & re-emerging MP3 players in the market (Fiio, Apple, Sony, many other audio-focused companies)
But then smartphones began to change themselves. They kept getting BIGGER and people no longer felt comfortable needing to lug around a phablet on bike rides, runs, or in their pocket while just walking around the office/house. So, "wearables" hit the market as a solution to wanting both LARGE phone screens and portability. Most people think "smartwatch" and "fitness tracker" when it comes to wearables, but the iPod Shuffle (being part of the "iSuite") might have been one of the first modern wearables.
I think we're going the wrong direction with the balance of of the size of the smartphone and the utility of wearables. Today's newest smartwatches are now coming with GPS/GLONASS, app libraries, and direct cellular connectivity and as a result they're getting VERY large. We don't want to carry around phablets while exercising, so we're taking other small devices and making them larger, separating out sub-devices for processes that were previously relegated to smartphones (The Mighty for Spotify playlists, Pebble Core for GPS/alerts, etc.).
But instead of a person owning 3 different GPS trackers, 4 wi-fi chips, and 3 cellular transmitters, why aren't we just focusing on further shrinking the smartphone has a hub for all the processes while developing a separate device to make the smartphone SEEM bigger? One such device is being kickstarted right now (Superscreen). This concept allows you to use your smartphone via a tablet-sized proxy.
Thus, you can get a physically small smartphone, carry it everywhere, connect everything you want to it (BT headphones, smartwatch, etc.) and when you want the LARGE form factor for reading, you pull the "big screen" from your portfolio, coffee table, or desk. Additionally, all of those other devices will need to charge less frequently because your phone is the one doing all the heavy transmitting.