In my experience you just don't keep as good a map of the codebase in your head when you have LLMs write a large part of your codebase as when you write everything yourself. Having a really good map of the codebase in your head is what brings you large productivity boosts when maintaining the code. So while LLMs do give me a 20-30% productivity boost for the initial implementation, they bring huge disadvantages after that, and that's why I still mostly write code myself and use LLMs only as a stackoverflow alternative.