What's the point with that quite lackluster story in Endwalker?

[linayar]

The term isn't solely used to describe female characters and nobody in this thread is taking issue with her on the basis of being a female character. There's also a male counterpart - gary stu - for the term, though often the two bleed together because they essentially mean the same thing.

Regardless, the term still doesn't seem to fit her character.

There is nothing in the sources that specifies what it was they were going to sacrifice....

Nothing that specifies it, but we still have the objection of Venat's group. If these are just simple lifeforms that they created and destroyed at a whim regularly, there is no reason for Venat to object, even if that solution was only temporary. And just because they objected to it doesn't mean they can't acknowledge the intention of Zodiark's group, so it's not an issue of not understanding why they want to call upon Zodiark, but the method, which is not only temporary, but also requires further sacrifice. Thus, they objected to it.

From there, it would be logical to assume this new life is the ancestors of now existing races, possibly even player character races.

The same reason that, despite him being a primal born of Salvation by a mostly peaceful race, he somehow has a demonic appearance. Whereas the opposing primal whose purpose was eventually to shatter the entire world(and is portrayed as a hero) gets an angelic appearance. Bias at its finest ;^).

That's just because they have the image created before the story. The fact that they still used Zodiark as a tool of salvation despite his appearance is more important than complaining about his demonic look.