I'm aware of Hack and it's quite nice, but I can't use it because a) my coworkers are … less open to new ideas … and will rightfully resist a "whole new language", whereas mgmt will see it as an attempt at "job security", and b) the biggest use-case for generics is third-party libraries (e.g. [0], [1], [2], [3]) which will never add support if this is not a core language feature.
[0]: https://github.com/doctrine/collections
[1]: https://github.com/schmittjoh/php-option