"Slaves gonna slave breh" is one answer, sure, but that's still one fukked up answer to give that character
like I said, Ymir doesn't move the needle for me but she got done dirty.
And I'd honestly have less issues with that being the reasoning if the explanation from Isayama wasn't "shyt, I don't know, y'all figure it out". Eren literally says he has no clue why it took 2000 years for Ymir to finally choose a path were there was going to be 1 random woman (Mikasa) overcoming her love and killing it....and somehow that was going to convince Ymir to let go of her love for Fritz, which would result in Titans not existing anymore.
It's one thing to leave things open ended and another one is to say "well, that's just because, you figure out the rest", imo