Bus factor. What happens if the knowledgeable lead developer gets into an accident? What happens if they retire? More realistically, what happens when they burn out because it's so hard to hire other team members and so they can never really take a vacation and have to take their pager everywhere with them? That's why businesses optimize for headcount.
I have yet to encounter positive solutions to those questions, as usually increasing headcount does not relieve pressure on team members - it's used to enable quicker replacement by yet-another-junior-to-burned-at-the-altar who will leave after gaining experience and burning out.
The classic "we have niche languages" solution to that is to enable on-the-job learning of those, and not just doing simple fact checking on whether someone honestly claimed to have learnt Popular Language X as claimed on their CV.
There's probably a link to monolith and distributed systems here. You gain way more resiliance when you get more people in your team, but what was once a thought firing in the brain of someone is now two people talking, which is order of magnitude slower. But on the flip side, you can probably handle more things in general this way.
