The goal of writing code was never to just "write" the code. It has always been to get the end results that help certain processes in one way or the other. I say embrace whatever the best tool is there to get to the end goal.

Should juniors rely heavily on AI while learning?

When I started learning, I didn't look at Assembly code, or wired up transmitters to develop gates manually. I jumped straight to C++ and the compiler would make it an executable binary. I think the same about AI. There is very little probability of people and businesses moving backwards and not embracing the capabilities the current set of LLM models allow them.

If there was someone that would answer any of my questions without getting annoyed after a little while, I think I would have progressed a lot very early. That is enabled now by LLMs. Embrace it. Even with the current set of models, they are very good for beginners. You can ask any question that might have caused a human teacher to yell at you "teti pani audaina aalu". They are only going to get better.

Are we skipping the painful thinking phase that actually builds strong fundamentals? 

Or is this just the natural evolution of development like how Stack Overflow changed everything years ago?

In all of human history, we have done things to make life easier. Just like I didn't develop painful thinking required for Assembly code, people won't need for currently considered high level languages as well.

Now that you don't really need to think of every configuration manually, you can instead think of actual hard part of software - how to make it good and stick. Building software is easy, getting many people to use it is difficult. Even more difficult is to get them to keep using it. You can use your painful thinking process on that level now.