It seems that
milch was used more often in the 19th and early 20th centuries than since then. These days the word is used mainly in India, Pakistan and Sri Lanka, with its literal sense of describing an animal used to provide milk. In other places, when the word is used, it's often in the context of a
milch cow being a metaphorical term for a source of easy money. E.g. from Scottish paper
The National in 2017:
Since the days of Franco, Madrid has used the Catalan economy as a milch cow to fund the rest of Spain
The word isn't used often enough and widely enough to warrant remaining as common.