> I have never once told my manager “it would be really nice to have a few junior developers. It would really help us get this project done on time”. They do “negative work”.

I have. A good junior can do in a week what a senior with domain knowledge can do in a half day, with only an hour of mentoring along the way. This isn’t a great exchange rate per dollar (juniors are cheaper than seniors, but not that much cheaper) — but seniors with domain knowledge are a finite resource, you can’t get more of them for love or money, while juniors are fresh-minted every semester. The cheapest way to shipping may not go through juniors, but the fastest way usually does; and that’s completely ignoring the HUGE side benefit of building seniors “the hard way,” which is still easier than hiring.