On grandma's car, I probably wouldn't worry about a brake fluid flush. On a truck that I tow a big RV and carry my young family around in, a full flush every few years isn't a bad idea. Brakes are kinda like gaining weight, the change doesn't happen overnight, but degradation does happen over time and can be hard to notice day to day.
Remember, Brake fluid is hygroscopic, which is a fancy word for absorbs moisture. As water is absorbed, the boiling point drops significantly, and brake performance is affected. If water worked well as hydraulic fluid, we'd fill all new cars' brake systems with water instead of brake fluid.