Yeah, the “but villains don’t die in One Piece argument!” is weak
This is a Marineford level arc, it’s the biggest thing that’s happened in One Piece since then. 400 odd chapters are leading up to it, the first time the Strawhats beat a Yonko. That’s enough of a paradigm shift for us to know that Oda, like in Marineford, can change up the usual stakes.
As for Kaido dying when no other villain has died before- no other villain has been so revolved around death. Kaido is introduced trying to kill himself. Throughout the arc, it’s not talking about defeating Kaido, but killing him. The man himself a few chapters talked about how it’s his death that will define him.
And if he is to die, which seems likely, I don’t think Oda would want Luffy to do it. Luffy’s always had a sort of innocence about him, and him killing someone could change that and take away a part of what makes Luffy Luffy. Now, I could be wrong, and maybe that’s a part of character growth Oda wants for Luffy going into the finale, I’d have no problem with that at all. I just don’t think that’s what he’ll do though.
Whereas Zoro does not have that innocence, he’s always been the harsh voice in the crew, and is the only confirmed killer in it (Robin could just have been trolling Usopp when she said she specialised in assassination).