You're completely correct. However I have a theory for the backlash, one I would like to reiterate.
I see a similar situation with Viktor's lore update that had his fans all riled up, and we all remember how that went.
I believe that the causes of this backlash are the same: They were generic yes, but not to the eyes of the fans, they were alive and real to them and the lore update threatened to make them into something that they were not thought of as before.
Unique yet Generic
Here's an explanation. Say that I'm an Ahri fan. Besides her vague origin story and her defined motivations, she's almost a blank slate in terms of the deeper aspects of her personality. I could imagine her as a seductress, I could imagine her as a struggling monster trying to be human, I could see her as a famous popstar in Runeterra. What Riot has given her before was generic yes, but they gave her enough detail so that she seemed real to her fans, and her genre appealed to many varied people, so this is partly why:
A. She has such a large and varied fanbase. A champion like Camille is very defined personality-wise and lore-wise, so only people who identify enough with her will main her. (After her nerfs). Ahri on the other hand isn't as defined, so her mains and fans fill in the gaps how they want to, and she has just enough detail in her lore to fit snugly with their headcanon and work.
B. Her fanbase is quite concerned for this lore update, because it threatens this relationship between being generic and unique.
However the tool Riot has used for her uniqueness is unstable, and it is what you mentioned.
Unique Only in Name
It's pretty obvious by now that Ahri's only special because of her looks and her uniqueness via rarity, but if we dig deeper we see her story arc is quite common. If you want to see another champion in the game that struggles with her humanity, look at Shyvana. If you want to see another protector, look at Ivern. If you want to see another attractive champion, look at Evelynn.
Yes, she takes all of these traits and puts a unique spin on them, but save Evelynn (who's getting a rework anyway), take these traits away and look deeper then you have a blank slate.
Ivern is the most heartfelt example of redemption and penance I have seen. He was a bloodthirsty marauder who learned the error of his ways, forced to come to terms with all the suffering he caused and spent years upon years in emotional turmoil and wonderment at the value of life, and lives now to spread life, no longer to bring destruction to it. From his VO and his lore you can see his personality.
Shyvana's lore has come out recently, so I imagine it's all still fresh in the mind, but her lore and color story show her personality well.
Ahri doesn't have anything yet, but what comes out can conflict with the headcanon of many varied fans; this lore will make her more defined and more unique yes, but the result will end up pushing some mains away.
But yes, being labeled a Vastayan will not make her any less special. Surely her fans want her to have a family.