LED's do lose output. But it tales a long time. Replacement isn't like we are used to -- complete failure. They have a stated life span identified a "L" which is 70% output as new. This ought to be available on the specs. For example a 30,000 hr. bulb will last 3750 eight hour days until a 70% dimming. It will regress faster after that, but still...
Brightness, measured in lumens, is one part light perception; the other part is color temperature. ... It is measured in degrees Kelvin. See diagram. In fact, the Kelvin scale goes backwards: The higher the color temperature, the cooler light gets, and the lower the color temperature, the warmer light gets. If you are trying to assess color in a specific setting, look at both. Adequate light is one thing, but human viewing can be something different. My preference is to get the highest number of lumens within budget but with the Kelvin temperature about 5500 degres K (daylight) which is close to (but less than "blue sky" outdoor daylight. I favor multiple light sources at the right temperature rather than lumens alone. Kodak real film is calibrated for 5500K daylight as an example. Artists favor North Light for their painting. Also LEDs have little or no UV so they are terrific for display and mort museums have switched to LEDs to protect their artwork.
View short video about lumen loss.