“Firstborn of all creation” is a partitive genitive: firstborn among creation, a member of the class. “Firstborn of Israel” doesn’t mean Israel is not an Israelite. Doesn’t the rank argument fail to escape the grammar?

The partitive reading is a serious grammatical option. The reply is that the genitive can also be read as a genitive of subordination (firstborn over all creation, as the NIV renders it), and that the choice between them is decided by context, which is why verse 16 matters: a for clause explaining the title by saying he created everything is a reason for supremacy over creation, not for membership in it. This is genuinely debated, and the grammar alone does not settle it; the sentence does.