Full disclaimer:I am simply sharing my opinion based on my experience with my BMW X5 3.0i, I am not responsible for anyone attempting to not change their brake fluid for the life of the vehicle.-------
Thats out of the way, I just wanted to post a brief commentary based on my vehicle hitting 205K miles. I had been running a sort of experiment, I do about 80% of all maintenance on my SAV. Only exception to that it is when I had my transfer case replaced, because I do not have the equipment or tools for major repairs. My engine and trans are still original and in excellent condition, except for an oil leak in the oil filter housing gasket and oil pan. I replaced my Front brakes and rotors with BMW original and my rears with aftermarket. Will save you a few bucks while still giving you that BMW brake bite one is accustomed to. So to the point of my post, I have not replaced the brake fluid since the car was purchased From a dealer as a CPO in 03. I would guess the stealer never swapped it out either. I have lived in Las Vegas,N CA and S CA for the past 10 yrs, to give you an idea about climate, taken it to the snow briefly several times and Vegas gets pretty good extremes for weather, more heat than cold. I cannot speak to Wisconsin or Minnesota winters, as that may affect fluid performance and characteristics. But for the rest I am going to say in my opinion it is not necessary to ever replace or flush your bmw brake fluid. Unless you get a leak or contaminate the fluid by opening the reservoir. My brakes still work as good today as they did when I bought the truck. I still have the original calipers and brake lines, was thinking of replacing those soon as they look weathered.
If someone can suggest a facility I can send a small fluid sample to evaluate the condition of my fluid I would be happy to. Im here to share what I think was a myth propagated by bmw and other auto manufacturers for decades to scare people into performing unnecessary maintenance. I feel my vehicle has provided a fair test platform to merit this claim as I drive it every day. If anyone has a different experience please post And share your thoughts. If you just want to troll dont bother Im simply sharing my experience given the listed circumstances.