I think there are costs beyond having to sacrifice writing code yourself. When prototyping yourself you learn a lot about the problem, see what design decisions lead to what tradeofs. While you write code your brain is always running in the background, giving you thoughts about how things could break, where the structure could be simplified, where the code could be extended. I feel this is lost or at least reduced a lot when an LLM writes code because you have a lot less contact with the software.