[Quoth]

I think we have a different definition of EOL.
If you mean they are not updating the Firmware for the PW3, then it's too soon to say. Mine is currently at 5.12.4 and is fine.
The DXG hasn't got KF8 support, it's 2.5.something and the older Kindle Keyboard is 3.4.something, it does support KF8. Both still access the Amazon store, one on 3G and the other WiFi. I read the DXG may not have the memory for KF8.
You can still get less than 1 year old replacement batteries for both.
Both work as they did originally.
OTOH many makers have turned off servers or even deliberately bricked stuff not very old. The iPhone 4s was dumbed by Apple ages ago removing the Apps for it. So it only has whatever apps where on it before then. Still connects to the Apple Store!

Another definition is "when they don't sell it". They've not sold the PW3 for a while?

Unlike Adobe products or Microsoft, "security vulnerabilities" isn't a big issue. Even there, while MS doesn't sell Windows 7 and no longer does retail support, there is paid and embedded support for years yet, probably.