What this demonstrates is that one can have a much safer programming language while keeping the key attributes of PHP that make it great: each web request having isolated state, concurrency model, and programmer workflow (ref. https://slack.engineering/taking-php-seriously-cf7a60065329).
Yes, I'm not necessarily advocating for any web developer to start building in Hack now -- mainly because it lacks PHP's rich ecosystem. The good folks working on the PHP language may be interested in these changes though.
For server-side web programming, what shall one adopt?
Python and Ruby are great dynamically-typed languages, with rich ecosystems, but don't have any type system. This makes maintenance, and evolution of the codebase, tricky in the long run.
For languages that do have type-systems: Go's is a bit too simplistic, while Java/Scala have robust type systems but are rather slow and don't hit the programmer workflow sweet spot.
There's no silver bullet around.
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?