Have you heard of Dwarf Fortress?
It's a highly detailed free game being written by one man who lives off donations so he can code full-time.

Why does that matter?

Because he released the source code.

Sort of.

He released a stripped down version with almost nothing in it with a tiny bit of a game attached on top(it's called "battle champs" and is like randomly generated risk). BUT what happened next?

Someone ported it to SDL and incorperated a modified bit of openGL stuff that greatly improved the framerate.

Tarn was impressed at the results and HAS INCORPERATED IT INTO THE GAME. SDL allowed it to be more cross-platform, so it went from windows only to having a release for windows, linux, and now mac.


Maybe a partial release might not be the desired outcome, but it would be better than giving away all of the secrets.

Why not release an open source version with an empty overworld with only one town and one cave in it, with all of the hidden fun stuff removed, leaving only the basic gameplay components?
Take out the specific parts that make ADOM ADOM, leaving the general code in, and see what happens?

Like everyone has already stated, it still is a choice that has to be made, this is only another option.