I see what you're saying, and question the assumption that a higher percentage of Hack libraries will be high quality. You raise a valid concern, and also this

I see what you're saying, and question the assumption that a higher percentage of Hack libraries will be high quality. You raise a valid concern, and also this issue cuts both ways. The FB libs have a good chance of being well-maintained. This will be true for PHP and Hack, alike. The rate of library neglect will be approximately equal between the two languages. The real question is: What does the data say about project / library neglect? I'd guess neglect rates may correspond inversely with language popularity. Languages with fewer people writing code in them will have more opportunity for abandonment and neglect. Who wrote and is relying on the library seems like a better candidate for predictor of proper future maintenance compared to language flavor.