The role of experience is often overestimated in software shops. It's a management crutch intended to overcome their own ignorance. They don't understand software, but they (think they) understand experience.
The reality is a really experienced dev is not really going to be all that much more productive than a well-managed and mentored junior dev. Sure, there are 10x devs, but really, sheer coding productivity isn't worth as much as it used to be. The 10x dev is going to quickly run out of things to do in most shops.
So what you have here is that sure, a junior dev can be a full-stack dev. He's dividing all his attention between all areas of the stack. He's not massively productive in any of those areas, but he's a hammer you can throw at software issues and be able to expect resolution.
That's what businesses need. Developers that can work on teams. You need just enough experience on the team to where problems one dev can't fix efficiently can be escalated to a senior dev, but there's no need for anyone to 'master' anything. Technology just doesn't work that way. You build your experience as you progress through your career, and you're always learning something new.
In fact, junior guys can be way more productive than senior devs. While the senior devs are thinking heavily about architecture, (while playing ping pong) the juniors are knocking out tickets.