And if developers were working on the old features instead of adding new ones, perhaps they would have found and corrected those old bugs. Then you could say that the code was "mature." But improving old code is not exciting, and it doesn't get attention.
Simply put, this is where LLMs are good, they don't have an ego.
But they do have a cost, and somewhere a human is still making decisions about spending that money on old bugs vs. new features.