It was a great learning experience though and I'm glad I did it, both in terms of learning how to write grammars (I used Clojure's Instaparse) and interpreters, and in learning about the users needs and wants.
But I shudder to think about if anybody else would have had to use it...
[1] Some interesting (IMHO) technical details: it was a synchronous language (of the transformational system variety), meaning that while it was executing, time was essentially frozen: it would gather all of its inputs, then evaluate the code as a pure transformation of its inputs (made it very easy to interpret!) and then write its output. The inputs are essentially immutable while the evaluation is happening. This was run in a database transaction for consistency. Each individual script was basically stop-the-world non-concurrent, but the scripts were independent and many could execute at once. I'm a very big fan of event driven synchronous languages.
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.
Overall, I completely agree with you, but I do find that HCL seems to be a pretty good fit for what its used for (at least for Terraform, which I have more experience with than their other tools).
My little story was really just me dumping my thoughts after reading about Stevecode, as someone who made a little language (not a configuration language though) and isn't an argument for doing so at all. I'm definitely in favour of sticking with an existing format for configuration (personally, I like to use TOML or EDN for my own projects' configuration needs, or JSON if its not normally meant to be edited by humans) and even for scripting, normally I'd stick with Lua or Javascript. My language came into existence because neither of those was sandboxed in the way I wanted (it was also deterministic and guaranteed to halt as there were no unbounded loops).
The whole point of configuration is that it's not code.
I use a simple litmus test: can your system work at least to some extent with empty configuration file? If not, you went too far and need to simplify.