Older LCD screens primarily used CCFLs for backlights, and these were notorious for going dim after only a couple years.

All modern screens use LEDs for the backlight which are much more robust but still get dimmer over time. I know that my screens certainly have dimmed significantly since they were new, but not so bad as to be hard to read.

Environment plays a huge factor as well. All screens appear dimmer in the brightly-lit room as opposed to a dark or dimly-lit one.