Some intreresting things said here, but the main reason to change the fluid is as it retains moisture the water will not compress like true hydraulic fluid and the brakes will not be as effective due to not pushing as much hydraulic fluid thru the system.
I have never boiled brake fluid on a street car, only on the track. If you are seriously concerned about boiling your brake fluid go to DOT5 silicon racing fluid as that is hydroscopic and does not boil until over 500 degrees but has other issues and that's way most dedicated track cars change fluid often.
Also most DOT approved, in fact all of the ones that I can think of have corrosion inhibitors so to rust your brake parts internally you'd need to run a long time with a lot of water in the fluid and you'd really notice braking degradation I would hope first.
Do it every 2 years, it's not that costly and easy to do if you are somewhat handy.
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