Well, that's kind of the point: Despite the age, OS-updates to it shall make it work just like any new device, except for being slower. If features (that are also present in the older device) don't work at all instead of just slow, that's fragmentation.
You should upgrade an old device because you need a new feature or you want more speed a modern device can offer you, but having to abandon the device because it doesn't work properly anymore should not be an option you're forced to consider, and that's the point, the vendors do force this upon you to drive their sales.
You should upgrade an old device because you need a new feature or you want more speed a modern device can offer you, but having to abandon the device because it doesn't work properly anymore should not be an option you're forced to consider, and that's the point, the vendors do force this upon you to drive their sales.