"Companies with relatively young, high-quality codebases benefit the most from generative AI tools" - this is not true
The codebases that use the MOST COMMONLY USED LIBRARIES benefit the most from generative AI tools
The codebases that use the MOST COMMONLY USED LIBRARIES benefit the most from generative AI tools
That means one might find themselves using deprecated but still supported features.
If LLMs came out during the Python 2/3 schism for example, they'd be generating an ever increasing pile of Python 2 code.