This is silly. Few places seriously expect a junior developer to be familiar with new frameworks. They expect to get a Java kid fresh out of school. And the fixation on
getting things done rather than careful consideration can--and does, I did it as a junior, it's part of becoming not-a-junior--leads to
getting the wrong thing done. Spending a week moving mostly sideways is no different than spending a couple days thinking and discussing and a couple days moving mostly forward, except you might make more progress depending on exactly how far the cowboy missed the mark in their haste to
just write code, bruv. (And the cowboy will miss. Everyone does, even if you're a genius. Not that groups of developers can't miss, too--but after a while you start to realize that they usually miss much closer.)
The role of junior engineers is that they are teachable and can generate value while they are being taught; the role of senior engineers is that they are teachers while both generating value of their own. There is no rarity to junior developers, there's potential plus inexperience. And that's it.