The main reason for changing the brake fluid every two years is that absorbs moisture from the atmosphere and once the moisture content exceeds a certain level and subjected to temperatures exceeding 100c then that moisture will boil, forming steam “pockets� in the brake system. Liquids are not compressable but gases are, that compression is provided by brake pedal action leaving you the driver with a pedal that goes nearly to the floor without having any braking effect!
No matter how long you leave changing the engine coolant it is unlikely to cause you and your passengers any physical harm as in the ancient brake fluid scenario, however , serious damage can occur to your finances due to corrosion within the cooling system (long term) or more catastrophically, a frozen engine (short term) , draining and flushing the cooling system then refilling with a 50/50 mix of water and antifreeze will help address the above and should be carried out at two year intervals.