I get your point - but also, the opened bottle has been exposed to the atmosphere - and if you were to use it to top up within the 2 years of a brake fluid flush, then that would be fine - at least as good as the fluid in the system, but if you use it to flush a system, you have no idea of the true moisture content, so don't know for sure how long it will be before it needs changing again.
An unopened bottle is a different kettle of fish - in theory that should last a good while.