Wouldn't that be cool?
I could imagine some pretty cool-looking primal weapons from that fight.
There are always easy ways to get around that though. After all, how are primals born but through faith and an abundance of Aether? Something this "last" Siren has in spades. Being the last Siren means there are no others to contend with, and therefore being the sole Siren, her "followers" like Skyfryn, and the many others who've fallen victim to her song are still obsessed with her. Heck, even Davyd, the quest-giver was smitten by her, not to mention the whole shore is ridden with fiends and beasts who are bound to her, so much so that they revere her even in death. Enough faith, and worship so close to a copious reservoir of Aether provided by Pharos Sirius could easily give birth to a primal.
It would not only do justice for her lore in other Final Fantasys, but give her, her much deserved spot in the primal spotlight.