I saw it as a response to Kaido wanting to go out in a glorious legendary way like Roger, Oden and WB but instead went out in an unceremonious fashion. The citizens of Wano didnt see how he fell, they didn’t even know a battle was happening (plus we never got a finish to an arc villain like this iirc). This embodies not giving the bad guy what he wanted. Big Mom was his partner in crime and a boss fight for Luffy’s rivals so I also don’t have an issue w her dying alongside him. She looked out for Kaido since he was a rookie, they're a dynamic duo so they fall together. Now if they’re both alive *then* I’d be annoyed at the writing choice. Also I don’t give the volcano all the credit; Luffy/Law/Kid did their damage to soften them up, knocked them out to a point where the idea of them trying to resist the lava would be futile.