At a high level think of Hack as Typescript for PHP. Although they completely replaced some data structures (i.e. arrays) that couldn't be made performant in their PHP implementation. Just like their customized version of MySQL, Facebook is stuck with Hack. It's too ingrained. Their customizations to MySQL became very specific to Facebook and they pretty much stopped open sourcing changes. Hack has taken the same path. They will never outsource Hack fully because some of the customization are only applicable to them.