You can freely mix Hack code and PHP code on the HHVM, calling functions and classes defined in any PHP file inside you Hack app. Then you can slowly evolve the codebase to contain more and more Hack and less and less PHP.

That's the huge point that nobody seems to see.

But then again, considering how many other huge points PHP developers seem to miss in general, maybe it's better to just let Node.js eat the world :)