I'm not sure anyone is debating that in the terms of PHP7 vs HHVM. Hack has been a different language for quite some time and that is exactly the problem. The more it diverged from PHP the less sense it made for most people to go that direction.

Also on a personal level I'm really not sure why anyone would want to go that direction. Of all the problems people usually say they have with PHP, just fixing the type system is not even close to #1. If Hack then really is a different language and you don't like PHP, why not migrate to something completely different?