[johnnyanmac]

>which “non-revenue generating” work (e.g. tech debt) actually needs to be done

This mentality is part of the problem. You can't fundamentally treat juniors as a profit center. Especially in more niche fields. They need to be trained and schools don't cover your specific pipeline.

maybe it doesn't "need" to be done, but some refactoring work will pay off to turn your juniors today into seniors tomorrow. If you can only think in productivity charts, then you don't hire juniors.