Brake fluid is hygroscopic which means it absorbs water. Humidity in the air can be drawn into the master cylinder. (Water molecules will go wherever the hell they want as long as they can find a path.) Eventually, this helps turn brake fluid corrosive. One thing I always have to test for in brake fluid is copper content and I do that by dipping a special test strip into the reservoir. It’s an indicator that corrosion inhibitors have depleted in the brake fluid. If copper molecules can travel up to the reservoir, water moisture can travel down the fluid into the calipers. Another thing to think about is that the fluid does move through the system. Every time you pump the pedal, you’re creating some kind of flow and/or turbulence in the system. Then as pads wear and caliper pistons adjust position, more of that fluid gradually moves into the caliper bores. One way or the next, your brake fluid is always gradually mixing with something that’s also gradually breaking it down.
In short, get a brake fluid flush done the right way when you need to.