I don't see how that invalidates the metaphor. For any version of code you can evaluate clean-ness. Writing is also not immutable, professional authors make hun

I don't see how that invalidates the metaphor. For any version of code you can evaluate clean-ness. Writing is also not immutable, professional authors make hundreds of versions before publishing. Of course books get 1 major release. So while ability to maintain and change is much more important in software, that is by far not the only factor for clean-ness, and I'd say hardly the most important.