Coolant can break down and loose effectiveness. Keep in mind what coolant does,

- raise boiling temp (So water does not expand and cause too much pressure in cooling system)

- corrosion inhibitor (Different metal like aluminium and steel does not like each other. Google for battery corrosion- This has nothing to do with battery but science)

- lubrication for water pumps (If rust do get into water, it becomes sanding agent and coagulant. It could build up in rubber seals drying it out and wear out water pump seals.)