In law, this sort of thing already happened with the rise of better research tools. The work L1s used to do a generation ago just does not exist now. An attorney with experience gets the results faster on their own now. With all the pipeline and QoL issues that go with that.
Note though that not all companies see it this way - the telecom I work at is hoping to replace senior onshore developers with junior offshore ones leveraging "GenAI"! I agree that the opposite makes more sense - the seniors are needed, and it's the juniors whose work may be more within reach of LLMs.
I really can't see junior developer positions wholesale disappearing though - more likely them just leveraging LLM/AI-enhanced dev tools to be more productive. Maybe in some companies where there are lots of junior developers they may (due to increased productivity) need fewer in the future, but the productivity gains to be had at this point seem questionable ... as another poster commented, the output of an LLM is only as useful as the skill of the person reviewing it for correctness.
I find a lot of the AI discussion seems to land in the "lump of labor" fallacy camp though.
Notes: https://simonwillison.net/tags/ai-assisted-programming/