The BSD operating systems sort of did the Ship of Theseus with Unix: it started out as extensions to Unix but by the early 90s it only required a relatively small amount of code to be replaced in order to be completely free of the original copyright. Beyond that I think your “de minimis” argument is right: not everything is copyrightable, some things are too small/simple. By analogy: an author doesn’t get copyright over every single word they use in their writing. (Though credit for inspiration might feel appropriate in any case.)