I've heard that tires that are actually being used on a vehicle over that period of time are not as susceptible to deterioration as tires that just sit around in storage because the heat cycles a tire goes through when it's driven on serve to keep the chemical composition of the rubber from breaking down as fast. The same principle behind a used car usually being better off in the long term even if it has higher mileage from being driven than one with lower mileage because it's sat stationary over an extended period of time.