A few years ago I ordered a bag of cheap warm-white LEDs from aliexpress and turned them into a floor light. Half a year later (and being on most of the time) they were quite a bit dimmer. Originally I suspected to have run them with too much current.
It seems that there are others with the same problem with cheap LEDs:
Summary: The light output of a LED string dropped from 67 lux to 7 lux over 9000 hours.
This source states a minimum lifetime of 50.000 hours before the light output drops to 70%.
Twice that should be easily possible by not running them at full intensity.
We have that issue with the bulb in the computer room, it's been running for around 5000 hours and is noticeably dimmer than it was, the previous bulb in the same fitting did exactly the same too. They are not cheap bulbs either.
The fitting is a simple screw in one with no shade so it can't be overheating, it tends to be turned on at about 10am and off again at 2 or 3 am so it runs nearly all day without being flicked on and off. it runs on the same circuit as the two computers, microwave and a few other lights. (230W 10A) we don't seem to have the issue with other bulbs, in other rooms that are not always on. But in our previous house we had 6 small "hallogen type" LED's in the computer room and they also went dim it seems in my experience that LED's don't like to be constantly on.
Life just hasn't been the same since the volcano erupted and now the air is full of tiny ads.