There are limits how badly can such senior screw up, or more likely forget some corner case situation. And he/she is on top of their own code and whole codebase and getting better each time, changing only whats needed, reverting unnecessary changes, seeing bigger picture. That's (also) seniority.

Llm brings an illusion of that, a statistical model that may or may not hit what you need. Repeat the question twice and senior will be better at task the second time. LLM will produce simply different output, maybe.

Do you feel like you have a full control over whats happening here? Business has absolutely insatiable lust for control, and IT systems are an area of each business that C-suite always feel they have least control of.

Reproducibility and general trust is not something marginal but core of good deliveries. Just read this thread - llms have 0 of that.