LLMs have not improved at all.

The people training the LLMs redid the training and fine tuned the networks and put out new LLMs. Even if marketing misleadingly uses human related terms to make you believe they evolve.

A LLM from 5 years ago will be as bad as 5 years ago.

Conceivably a LLM that can retrain itself on the input that you give it locally could indeed improve somewhat, but even if you could afford the hardware, do you see anyone giving you that option?

You cannot really compare the two. An engineer will continue to learn and adapt their output to the teams and organizations they interact with. They will be seamlessly picking up core principles, architectural nouances and verbiage of the specific environment. You need to explicitly pass all that to an llm and all approaches today lack. Most importantly, an engineer will continue accumulating knowledge and skills while you interact with them. An llm won't.