Ship of Theseus is a great argument for crediting the original author, even if it's been 100% re-written over time: The ship would have started from one (or more) expert ship builders, while the continual patching could be done by comparatively-unskilled hands.

(Not that this is the case in software, but merely saying that that parable would lend itself to an easy answer as to whether to credit the original or not.)