Well, the compiler was its own thing and had very different goals. I'm sure that wouldn't have flown for most use-cases and from what I've read it was used only

Well, the compiler was its own thing and had very different goals. I'm sure that wouldn't have flown for most use-cases and from what I've read it was used only for running production code because compilation took a while. HHVM also adds a lot of syntax to make Hack. And moreover I don't doubt Facebook wanted the option to remove backwards compatibility in the future. Issues like MAX_INT and pass-by-reference don't seem pressing enough if you're using the language, but if you're building a compiler or VM small language quirks will be the one exception that break whole potential optimizations and/or ways of architecting things. Facebook is not concerned with backwards compatibility because they can just change their source code, but PHP is, in order to remain relevant to the community. I honestly think trying to work on the same VM would've caused more issues in the long-run.