Well to be more literal, Phoenix wasn't "nothing" as she merged with Selh'teus to defeat Promathia, but lore wise, in order to revive Phoenix (let alone 'give birth' to her) you had to give up 100,000+ human sacrifices, so lore wise it made sense why it was never realized in XI.

In XIV, I think given how Yoshida wants to do free company summoning system, it would actually be the perfect time to reintroduce her to the series, since for example in PvP if everyone is getting slaughtered, nothing like summoning Phoenix and dousing those bast-people in flames and reviving your allies.