I remember learning in school... LEDs should never die/burn out as long as the right amount of voltage is applied, anything higher than X amount burns em out. Then again this was back in Highschool with cheap lil LED red lights for Computers class...
Though one would think, it SHOULD apply to all LEDs. But, I could be wrong.
Besides a little loss of intensity, it's true that the actual LED diode can last this long. What I'm referring to here is not the diode itself, but the phosphors that are mixed into the resin lens which are responsible for emitting the white light. White LEDs are usually UV LEDs, in some cases blue or yellow emitting LEDs (monochromatic). Phosphors (a white powder-like substance) are mixed into the resin lens or the lens itself is coated with this powder which emits white light when UV light hits it (think white shirt glowing under a black light in a night club). Just like fluorescent bulbs, they generate UV lighting internally and excite the phosphors which are coated on the inside of the glass tube, these "glow" so to speak white. And it's these phosphors which degrade over time due to UV radiation, heat and other factors. So while our LEDs may indeed continue to work (emit UV) for the rated hours, there's the possibility that the phosphors still degrade as quickly as they do in other applications, meaning the spectrum and intensity of the white light would degrade regardless of the healthy state of the UV emitting diode.
We've tried replacing the drivers on one gang of LEDs with no change in light quality, so we know it's the LEDs that have degraded somehow. Obviously there are other possible causes, perhaps the drivers were operating the LEDs above their rating or a poor job was done in dissipating heat internally... but it still leaves me curious regarding the phosphors and how, if at all, they are able to extend their lifespan to bring them anywhere near the lifespan of an LED diode.