I thought it was because the ammonia in the adblue degrades after a couple of years and no longer does its job in the emission controls. Having said that, I wonder if you use 90% of it and have 10% dilution, won't it still work in controlling NOx?
Adblue is cheap. You can buy it at the auto parts stores and truck stops. Is it hard to drain?
On my first service, it was not clear to me whether the dealer drained and filled or just filled. I think the documentation implied the latter.