That's true. Presumably the junior developer is cheaper while they're junior, but if nobody's hiring them anymore then I guess they're not cheaper enough to compensate for needing to train them.

Maybe by creating a workplace where developers want to stay you could keep them around at slightly under market rate? Not everyone is looking to job hop every 6 months. But the amount below market rate that you seem to end up by staying in one job feels like it's unreasonably large.