Most LEDs run on DC, and the built-in transformer is the most likely component to fail. If the LED is failing and getting dimmer, it's most likely due to poor heat dissipation.
If we had little 12v adapters and separate LED modules, you could reduce waste by only replacing the part that fails, and manufacturers would have greater incentive to improve build quality. Instead, we get cheaply manufactured bulb-shaped disposable units that need to be thrown away when one part fails.