AFAIK the reason behind doing coolant flushes is it gradually becomes electrolytic over time. After a few years kahit visually malinis pa yung coolant it eventually becomes an electrolyte like plain water, which can cause galvanic corrosion especially if the engine and radiator use different metals (usually iron in the engine block and brass/copper in the radiator). Recommendation from Toyota in Vacaville CA is to flush coolant every three to five years or 30k miles.¹


¹ How often does your car need a coolant flush?