Antifreeze does a lot more than just lower the freezing point. It has corrosion inhibitors and various other additives that are necessary for a healthy engine. While the ethylene glycol that most AFs use will maintain its cooling qualities for a long time, the additives "wear out" over time and need to be replaced.
I don't know how long modern additives are good for but I like to replace all "long life" fluids in a car (AF, transmission and differential lubes, &c) every 30-50K miles/3-4 years or so. More often on older, "classic" cars. YMMV
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