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.)

You could equally argue that Gail's law is applicable. Without someone creating something simple but working it may never have existed.

I suspect you mean "Gall's Law" (see https://en.wikipedia.org/wiki/John_Gall_(author)#Gall's_law).

Thanks for the link. I hadn’t ever heard of the Ship of Theseus – though I’ve referred to Trigger’s Broom from Only Fools and Horses when discussing this concept with others: the character worked as a street-sweeper and claimed to have had the same broom for the past 20 years (however, he had changed both the handle and the head multiple times during this time).