The act of heating water will cause minerals to precipitate out of it...think of the bottom of your teakettle or coffeepot. This will happen whether it is gas or electric. If you install a full-port ball valve for the drain instead of what typically comes with it, you have a chance of cleaning it out periodically. If you maintain (i.e. replace) the sacraficial anode(s) on a regular basis, the tank will last longer, but if you forget and let it go too long, it will be shot. Don't think there is such a thing as an electric or gas fired water heater that will last forever. Depending on your water conditions, they can sometimes last a very long time, though. If never drained and rodded out, eventually when you have hard water, the tank will effectively become smaller and smaller and it becomes harder for the heat to get to the water from all of the mineral deposits. Even if the tank itself is intact, once this happens, you need to replace it.