So.. Are we sure that new brake fluid makes the brakes fell better and it's not just the fact the system is being bled of any air that may have gotten in?
Not discounting that brake fluid gets funky.. But fluid doesn't compress. Be it water or hydraulic fluid. Just thinking out loud here..
Well, it's kinda the same situation really. Brake fluid soaks up the moisture, making it funky fluid, and the wet boiling point drops, and drops, and drops until the fluid can't absorb anymore, and you have free water in the system. Boil that, and your pedal just gets soft as a sponge. So not you have fluid with a low wet boiling point, free water, and some residual gas from some of the boiled fluid.
Plus all that fluid rusts out steel pistons and calipers and causes them to seize...especially the rear SN95 type calipers.