Aerith was so memorable because of how they developed the character. You spend so much time learning about and building a relationship with that character before she's taken away. I would argue that Tidus's death (disappearance) was almost just as impactful. And really, he really did die at the end of X. We wouldn't of got an X-2 if the fans weren't so impacted by it.

SE just hasn't done a good job in fleshing out characters for the last decade or so (Zack may be an exception). They could possibly have another chance at devastating their audience in XV :P Depends if they're any good at making you love some of the characters in it. I have to imagine from the tone of the story peeps are gonna die.