I think the ipod being so common has helped it remain useful 20 years on. There's a lot of them being sold cheaply as people clean out old drawers, spares/replacements and upgrades are readily available, how to work on them is common knowledge and tools to work on consumer electronics is common, and rockbox is available for most of the range. MP3 players were a commodity good with a huge range of models available from practically any company capable, but most won't have the market around it. My ipod has a 128GB SD card instead of a HDD and a battery that lasts at least 5.5 days playback instead of whatever the original 650mAH allowed