It would be interesting to see some research on exactly how much sustained productivity boost programmers can get by using LLMs. The reason this is a bit complicated is that the code would have to pass certain quality metrics. In particular when it comes to structural soundness -- whether a piece of code is something you can build on and evolve, or if it is disposable.
I think different generations of programmers have different opinions on what is quality output. Which makes judging the quality of code very context dependent.
If I were to guess I probably get somewhere in the range 10% to 20% productivity boost from LLMs. I think those are pretty astonishing numbers. The last time I got this kind of boost was when we got web search engines and sites like stack exchange.
I would suspect that if people experience 100% or more productivity boost from LLMs, something is off. Either we have very different ideas about quality, or we are talking about people who were not very productive to begin with.
I also think that LLMs are probably more useful if you are already a senior developer. You will have a better idea of what to ask for. You will also be in a better position to guide de LLM towards good answers.
...which kind of hints at my biggest worry: I think the gen-z programmers are facing a tough future. They'll have a harder time finding jobs with good mentors. They're faced with unrealistic expectations in terms of productivity. And they have to deal with the unrealistic expectations from "muggles" who understand neither AI nor programming. They will lack the knowledge to get the most from LLMs while having to deal with the expectation that they perform at senior levels.
We already see this in the job market. There has been a slight contraction and there are still a significant portion of senior developers available. Of course employers will prefer more experienced developers. And if younger developers believe in the hype that they can just vibe-code their way to success, this is just going to get worse.