heh. Was that friend a PC user? my friends blame everything from network problems to computers crashing on macs, and make every probalem seem a hudred times larger than it is. Usually they're just joking around, luckily...
All LEDs become dimmer over time, but it takes several years (that's years of operation, not just sitting there not running) before they become dim enough to notice. Optical sensors can become less sensitive over the years too. The whole computer will be very old and tired by the time it becomes really noticeable.
Even if some nasty accident befell the backlight, the keyboard itself is pretty much the same as any other and should keep on working.