Yea and no.

Simply put, iPod had a price tag that was (for a time) interesting. But the device itself was limited in its features. Whereas iPhone was only ever slighty more expensive but had more feature. It made sense for the customer to migrate to the iPhone. Also, it was also a business decision to halt the development of the iPod.

For the eReader, the story is different. Firstly, it is not controled by a single company, so there are multiple business decisions making it stay alive. Then, a tablet and an eReader serve two very different purpose. One is for general use while the other is only used to read ebooks. Because of this, the tablet and ereader have two different types of screen : LCD/LED and eInk.

The first one is based on sRGB and will deliver colours flawlessly at the cost of more power consumption. And by design, the tablet has a large processing power. That means the battery will drain faster. Which is not something you want when reading a book.

On the other hand you have the eink screen which only covers a few colours and, as such, don't use much power. Plus, you don't need any computing power to display texts, so the device is very energy efficient. That creates a specialised device that can last for many hours before needing a charge. Plus the fact that eink makes it almost look like you are reading a real book and not a screen.

I think that covers partly your question. And sorry for the broken englisn.

Your phone can play music just like an mp3 player can.

Your phone doesn't have an e-ink screen.

That's the whole reason.

A dedicated e-reader is way better for reading than a smartphone or tablet. They use e-ink screens which are better for your eyes and for battery life.

Also there are dedicated mobile music players. A lot of them are targeted at the audiophile market and can drive high impedance headphones and have a high quality dac. But most people aren't audiophiles and don't use headphones that would benefit from something better than a smartphone.

Every iPod feature has been rolled into the iPhone. In fact with phones having direct access to streaming platforms like Spotify, phones have made iPods redundant. Nobody wants to sync with iTunes anymore.

E-readers however still have benefits over phones. The screen quality and the battery life.