Is the problem actually the presence of juniors, or rather the fact that in "agile" all developers are treated as replaceable, so the juniors randomly get assig

Is the problem actually the presence of juniors, or rather the fact that in "agile" all developers are treated as replaceable, so the juniors randomly get assigned strategically important tasks where they can do long-term damage? I imagine that after a month, when the senior developer(s) laid down the foundations, it should be possible to find work for the juniors (not all at the same time, but adding them gradually). They just cannot be assigned tickets at random.