The Googling you do to get an understanding of something you've never seen before can be done in a fraction of the time by AI.

A feature wasn't 10000 loc written by a AI before that no one except the AI with the context understands. If you review all that to understand it fully your productivity gain diminishes, the gain might not go away fully but it is much less when you can be woken up at 3am because of an incident and start reviewing everything.