LLMs may theoretically be able to generate millions of correct lines of code.
But for any important code the only way to know that it's correct is to hand-audit every line, which is harder than writing every line because this allows you to build a conceptual hierarchy and model within which you can think about the code.
So there's a fundamental limit to the speedup from LLMs when writing anything you care about.
(The main objection will be what about when you have a human engineer working on your behalf. But this is a distraction: sometimes you can fully trust an engineer, which is not at all true for LLMs.)