Given the big investment, I don't know what they would use instead. They seem to now have a very high level language well suited for the web, but with most of the advantages of a typed-from-the-start language, and great performance for the tasks it's doing. It's hard to think of an existing language that wouldn't be a downgrade in some way (productivity, performance, re-work) for them now.

Ironically, the only thing I could see reasonably replacing Hack/HHVM for them is...PHP. Assuming PHP has some success adding all the typing, async, etc, features. They already addressed the performance differences vs Hack/HHVM.

Note: All in context of what it's used for, and the investments already made. I get you wouldn't choose Hack/HHVM as a new company.