Lexicons wouldn't replace NIPs per se; they would replace Kinds. ATProtocol has protocol governance and lexicons as separate things, whereas on Nostr both are rolled together into this more amorphous NIP concept. So you would still need a NIP to define how lexicons work (how they are published, how they are propagated to become immutable, etc.) but what you would no longer need are those NIPs that do pretty much nothing else but define Kinds. In other words you would separate Kinds from "other governance stuff", and I can't see any technical barriers standing in the way there, just a question of desire.