I am a junior developer on my first job as a developer at a startup with around 50 developers.
The company, luckily, has a very opposite approach to hiring jr devs than what is described in the article. There are usually one or two jr developers per squad of 4 or 5 people (which is actually a maximum, the company is aiming only at more mid-level or senior devs right now).
There is a very collaborative environment, a lot of help from all senior developers - not only the ones in my squad.
What I think is missing in the economics there is that I (and all other jr developers) do provide measurable value to the company after 2 or 3 months. I am able to create simple, but necessary and demanded by the business, features by myself, just with senior developers reviewing my PRs.
This idea that junior developers are a drag to senior developers is fake in my (limited) experience. I make the senior developer in my team more productive most of the time, and after around 8 months in the job, we are actually hiring another junior developer because I think I am ready to take some of the responsibility of mentoring him/her on a basic level.
I am pretty sure that I am a net positive to the company on a $ invested per value delivered ratio.