EXACTLYYYY
I feel like a lot of people really want/wanted Kuina to be Tashigi because they can't accept that all the potential she had was ruined because she had a tragic accident.
What makes Zoro's backstory sad isn't so much the scale of destruction, like on Robin's, but rather how down to Earth it is, as Kuina tripped, fell and probably broke her neck in a bad landing. It's that simple, and it serves as a good contrast to the larger scale backstories.
Today I was showing my nephew the second episode of OPLA with Usopp, and we've got to the scene when Zoro was trying to get out of the well, while thinking about Kuina. My nephew complained about how in anime Kuina died in an accident which he found lame, and I started to think about it deeper.
I came to conclusion that Kuina's fairly normal death via tripping accident is meant to be doubly tragic. She didn't just die young in a stupid, easily avoidable accident, while having such grand plans as becoming the greatest warrior ever; she didn't even die in combat. All of this happens some time after Zoro and Kuina promised each other that one of them will become The Greatest Swordsman in the World, so Kuina's ordinary accident feels like a cruel twist of fate. She didn't even die a warrior's death; she just tripped.