The “dirt” is probably a very thin layer of metal (tungsten?) deposited on the inside of the bulb. This would be the same material as the filament. The bulb has lasted this long due to the lack of thermal cycling from turning it on and off.
The vacuum tubes in Cheyenne Mountain (NORAD) lasted so long for the same reason. The old vacuum tube system lasted until the late 80’s early 90’s
Yes, just wait until Obama comes in and turns off the switch by mistake.