Most phones, laptops and wireless earbuds are discarded while still capable of everything they were bought for. What has failed is the battery, and its decline follows a schedule set by chemistry.
Cells lose capacity as they are cycled
A rechargeable cell holds slightly less charge after each full cycle, through chemical changes that are inherent to how it stores energy.
Manufacturers usually specify a number of cycles after which a stated proportion of the original capacity remains, and that point arrives within a few years of daily use.
Nothing else in a modern device degrades so predictably, which is why the battery determines when the product stops being usable.
Heat and charging habits accelerate it
High temperatures shorten cell life considerably, so devices used while charging, left in hot places or run at sustained load age faster.
Holding a cell at a very high or very low state of charge for long periods also increases degradation, which is why some devices now cap charging until it is needed.
Fast charging generates additional heat, and the trade between convenience and longevity is real even where it is well managed.
Software masks decline before revealing it
As a cell ages its ability to deliver current under load falls, which can cause a device to shut down unexpectedly at what it reports as a usable charge.
Systems respond by limiting peak performance to stay within what the battery can supply, so the device becomes slower rather than unstable.
The user experiences a computer that has aged badly, when the component that changed is the one supplying power to everything else.
Replacement is designed to be difficult
Thin construction, adhesive assembly and the demands of water resistance all favour a cell that is glued in rather than clipped in.
Replacement remains possible for most products but requires disassembly, which raises the labour cost to a level that competes with the value of an older device.
Sealed products such as wireless earbuds take this furthest, since the cells are small, degrade quickly and are effectively unserviceable.
What extends the useful life
Avoiding heat, staying within a moderate charge range and not leaving a device plugged in permanently all slow the decline measurably.
Repair rules in several jurisdictions are pushing towards better parts availability and easier replacement, and the requirements differ by place and continue to change.
For a device that is otherwise working, a battery replacement is usually the cheapest available upgrade, and it addresses the component that actually failed.