Maybe in terms of pure productivity, but if you can match hiring to roadmaps you can give them more approachable/further from revenue work for which seniors would be an overkill. Etc. I'm just saying anyone with a simple explanation is only telling you part of the story.
Besides, hiring is expensive. If you say live near a university, you have an edge in finding talent.
My experience suggests there is no such thing. There is work that seniors might not want to do, but if you hired well then they will be professional enough to do it, if it really needs to be done. And it takes some experience to determine which “non-revenue generating” work (e.g. tech debt) actually needs to be done, to advocate doing it to the stakeholders and to actually do it well.
Juniors need a lot of supervision and that is not free. Which is not a reason not hire them in the first place, just that that it should be done mindfully.
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.