With thousands of litres of milk, it shouldn't be easy to mess up weeks of milk gathering with a simple misclick on the wrong cistern.
Please allow this quality of life feature to exist. I don't think there's a single player that would want this to be otherwise.
Generic milk is kind of garbage, and if people really want some, it's already very easy to mix with barrels.
Same thing would be nice with honey. Nobody wants generic honey, and if they do, they have barrels.