I don't think it'll kill off the Panamera, iirc, it's volume is decent in Porsche's lineup, it can cater to separate markets. Taycan's interior volume is smaller as mentioned, it's more of the size of 3 instead of S, or civic instead of accord, or whatever, and the jump from that to Cayenne is a large leap.
(Also not everyone can or want to plug in still)
(Also not everyone can or want to plug in still)