While i dont disagree with the larger point here i do disagree that all the code we ever need has been written. There are still soooooo many new things to uncover in that domain.
New cryptography algorithms, particularly post-quantum cryptography
New zero-knowledge proofs
Video compression
And so forth.
Those are all instances of reuse of existing techniques in new contexts. And when genuinely-new algorithms do arise from genuinely-new areas of study, it's easy enough to teach LLMs how to apply and deploy them.