I can't really speak to the impact on junior devs since I haven't worked with any since the start of AI dev tools so this comment is kind of off topic.
Totally agree that should be treated as learning tool just as much "give me something that works" tool. If Junior devs are not taking advantage of that side of it instinctively out of their own curiosity and interest, well, maybe they were never going to be good developers in the first place even without AI.
What I can say is that for me as a as senior dev with 22 years experience who has been using using these tools daily for about a year now, it has been a huge win with no downsides.
I am so much more efficient at unblocking myself and others with all the minor "how do I do X in Y" and "what is causing this error" type questions. I know exactly what I want to do, how to ask it, but only partially what the answer should be... and AI takes away the tedious part of bridging that knowledge gap for me.
Maybe even more significantly, I have learned new things at a much faster rate. When AI suggests solutions I am often exposed to different ways to do things I already knew, features I didn't know existed, etc. I feel good that I found a solution to my problem, but often I feel even better having learned something along the way that wasn't even the original goal and it didn't really take any extra dedicated effort.
The best part is that is has made side projects a lot more fun and I stick with them a lot longer because I get something working sooner and spend less time fighting problems. I also find myself taking on new types of projects that are outside my comfort and experience zone.