Humour me, why must there be such a threshold? If version B is modified from version A as a derivative work (therefore licensed as A) and version C is modified

Humour me, why must there be such a threshold? If version B is modified from version A as a derivative work (therefore licensed as A) and version C is modified from version B as a derivative work of B, then would C not still be licensed same as B, and therefore A? It seems it depends on how you view the accumulation of differences. If your only comparison is always against version A, then yes, eventually there must be enough accumulation of differences that one could claim a work is no longer derivative. But if you consider all versions between A and yours, then suddenly its just a series of gradual changes, none of which individually cause the work to become non-derivative. The Ship of Theseus parable seems to also fit this latter interpretation more -- it is after all about replacing items one at a time. I agree with you from a practical legal point though. My argument above seems to imply that there's only been derived work since the first cell split in two. In practice this would be a nightmare; There should be a threshold whereupon a work can qualify on its own merits.