The benefit is intangible and honestly if they didn't have the opportunity to pursue a PhD they wouldn't leave the country. They would do what the 99% of people like them who are unable to pursue a PhD in their chosen topic do, work another job and publish a smaller body of work in a less prestigious setting. The fact of the matter is that those are not the 1,600 people who have the ability to earn a PhD in English or Literature, it's the ones who's interests and personal profile afforded them the opportunity.

Now to be clear, I'm not saying that this work is unimportant. Intangible benefits are (despite the name) very real and do benefit the nation. It's just a much more complicated than engineering PhDs making stronger forms of concrete or whatever.