... simply choose not to die? The alternative being they continue existing either as a standalone soul (e.g Unukalhai, Cylva/Cyella) or an Ascian-like being (e.g Minfilia, ...Zenos?)?

Maybe they're not at that level of Echo "mastery" yet (like Zenos boasts before the Shinryu fight) but... if Minfilia could do it then I don't see a reason our own character couldn't, right?

And I say theoretically because anyone reaching the end of their natural lifespan is so hilariously irrelevant in this game where time just has no meaning. But I'd been wondering...