I feel like a good compromise would be to stick the code base for each big release into some sort of vault. Not open to the public, but for there to be an agreement that should Tarn ever be done with it, it can be released. Basically give keys (or copies) to Tarn's brother as well as a few of the DF community members that Tarn trusts the most.

This way it stays just Tarn's work of art for as long as he wants it to stay that way, but if anything did happen it wouldn't be lost to society.