You are correct regarding the rubber hoses failing internally. They will degrade over time and harden. Modern quality coolants are designed to not affect rubber hoses; however old coolant will become caustic and degrade the internal rubber surface of the hose. If you maintain a regular service interval of your coolant including reverse flushing the system every 24 months you will lengthen the serviceable life of your hoses. It would be fair to assume that a well-cared for cooling system should be reliable for at least five to eight years.