Thanks for sharing. There are often good reasons for creating DSLs, internal or external.

However I argue that I haven't seen many cases where configuration DSL were well justified.

By configuration here I mean something that describes differences between different deployed instances of the same code. If you take all deployed instances of the existing system and see what is minimal amount of configuration to describe these, you would usually find that very little information, just a few lines would be enough.

The rest of configuration file complexity comes from the designer's attempts to predict the future extensions. My point is, these should be resisted for the sake of future generations of maintainers.