i believe LED's can get dimmer over time, at least certain types/makes. I have read a few articles on LED TV pictures not being as bright over time, I forget what brand or reasoning, but I have heard that they can dim down over time.
This is correct, and for the exact same reason. LED TV's are backlit by white LEDs, that's the item that looses intensity over time due to phosphor degradation. Plasma TV's are 100% based on phosphorescence, hence their loss of intensity over time and shorter lifespan Vs LCD screes, which can usually have their backlight (LED, ELP or CCFL) replaced as the phosphors degrade. High-end LCD panels use RGB LEDs which don't use phosphors, providing longer lifespan and better color control.
I would think that if someone has developed a phosphor with a huge lifespan we would see this technology in phones, TVs, laptops well before our little niche market. Or do they not want a TV to last 20 years...