It is VERY reasonable.
First of all, warranty is not conditioning you to change brake fluid in dealership.
Second, brake fluid is hygroscopic. It absorbs moisture, when it does your boiling point goes down, when it goes down due to presence of moisture it can create "air pocket," and when it does you are out of brakes.
Third, it is cultural thing. Europeans, but especially Germans are obsessed with technical conditions of their vehicles. Every time you want to register car you need to pass technical inspection. One of the things they will check is boiling point or % of moisture (depends how each country does it). If boiling point is below 120c (as far as I remember when I was doing it) you cannot register vehicle. If moisture level is 3% or higher, you are also out (that is just brake fluid. There is slew of other things including how your headlight beams are set up).
So, when VW creates manual there is a LOT of cultural preferences that trickle down into it.
I personally change fluid every two years on all three vehicles using ATE TYP200 fluid. But then, I also have winter tires for all three vehicles etc.
It is your choice.