I think it's difficult to show ROI on junior devs to non software companies as there is a lot of work involved in training them up. And most normally stay with a company max 2-3 years. Businesses have expectations that senior dev level ability for juniors at their cost. Or the mentality that three junior devs should be able to provide the same output as a single senior dev.
Development should not be repetitive work, that should be automated or scripted. And the non repetitive work lies in that strange field of both creativity and logic that makes development such a rareish field. Sure , you could hire junior devs that are basic code monkeys , churning out boilerplate code but they won't learn and evolve if you just leave it at that.
Well, the ROI is that without hiring, a company is by definition dying. If you aren't growing, you are dying.
Personally I believe juniors are undervalued. You get some really bright college graduates who are eager to learn.
And if juniors leave as soon as they are good enough to get a better salary elsewhere, that is the company's fault, not the graduates. They should want to stick around because they are still growing and are earning decent salaries.