A brake fluid flush is normal maintenance that most people skip over or don't understand why they should do it. It's pretty easy to see that your diff gear oil needs to be replaced, but not as easy with 5 year old brake fluid. It may look new, but it's absorbed a lot of water each year. Combine that with how a brake hydraulic system works and it's easy to understand why it should be replaced. What I mean by that is brake fluid transfers pressure from the master to the calipers/wheel cylinders which causes the fluid to go back and forth each time you press/release the brake pedal. I like to think of the brake fluid as Listerine that gets swished around. The fluid may not be moving the entire length of the system but it goes back and forth over the same seals thousands of times which picks up microscopic debris that can eventually cause accelerated wear to seals and sensors.
TLDR: Change your brake fluid more often than you think you should. Use a test strip to test the water content too.
TLDR: Change your brake fluid more often than you think you should. Use a test strip to test the water content too.