Honestly, I think having engineers come and go is a net win for companies. I've worked at places where the "old guard" has been around forever and they are inevitably stubborn, obstinate, and convinced there is no other way to solve the problem. By changing jobs and environments, you get to see in practice that there are many different ways to implement a solution. Only seeing one set of systems (unless they are a FAANG, which is large enough to have the breadth to counteract it), would ultimately cause those architectures to stagnate, injecting new blood is also injecting new ideas.

I do however, agree with your sentiment, those junior folks that are getting all the work done only get rewarded with more work to do.