The thing about flushing brake fluid is to remove water in the system.
Let's say a bad system has 10% water by volume.
If you change 90% of the fluid, now you have an average of 1% by volume water in the system.
Since brake fluid is hygroscopic, water in the abs system fluid amount will migrate to balance out in the rest of the fluid in short time leading to 1% water all over. That's why in older cars I never had issues with just changing the fluid in the reservoir more often. Water in more saturated sections migrates over to drier sections. Of course in the bmw, I do flushes to change more of the fluid but not everyone has access to ista and my point is that changing a majority of the fluid is the key to preventing water buildup and corrosion.

In cases where water doesn't mix with fluid, like silicone dot 5, the water builds up in the low points as the fluid doesn't "hold" the water like our brake fluid does.