I have a large team that works in a 1.5M line codebase. Not everyone is familiar with every line, but we regularly make changes that would blow through the usable context window of LLMs (which is, in fact, much smaller than advertised).

Is a messy 1.5M lines of tightly coupled code best practice? Of course not. But it evolved over about 20 years and processes tens of billions of dollars of financial transactions. In my experience, it is archetypical of real-world software for a large successful company.

I use LLMs where I can and they're incredibly useful. But their limits are severe compared to a good human software developer and the shortcomings mostly revolve around their tiny context. Human neuroplasticity is still champion.