Are you using or have you used any of these AI tools for any length of time? Truthfully, no, I don't see it allowing 3 people to do 4 peoples' jobs, or for juniors to do the work of seniors. If I had to spitball a guess, I'd say there's potential for about a 10-15% productivity improvement on
some tasks. Overall, as a senior/staff level software engineer, I can see these tools boosting my productivity
maybe 5-10%. That's nowhere near "3 people doing the job of 4" levels.
Speaking of juniors doing the work of seniors, you've got it totally backwards. The people who benefit most from these tools are the more experienced, more capable people, not new grads fresh out of college. There are a ton of things a SWE does that no LLM will help you with -- at least not until virtual avatars get to the point where I can send fake me to a meeting and expect to still have my job at the end of said meeting. Being able to evaluate and understand the output of these tools is a key skill necessary for gaining a benefit from them, and that's precisely the skill that juniors, by definition, lack.
That said, this whole market dynamic of not hiring juniors at all right now is going to be a pain point for the industry at large in about 5 years if it keeps up. If you're a senior+ level SWE right now, that might end up being to your personal benefit, but it certainly won't benefit the tech industry when there aren't any mid-level engineers to hire, because we didn't cultivate, mentor, grow, and train them as juniors today.