I don't see what it fixes. You're just replacing an interpreter and in all likelihood bundling a web server.
PHP 7 is a pretty solid language. I don't like WordPress at all, but there's so much cruft, legacy nonsense and plugin garbage that you won't win that battle.
well obviously we won't win the performance battle, and it may be impossible to protect the database and user data, but sandboxed execution with a virtual filesystem could protect the overall system, keep it from joining botnets and sending spam, etc.
PHP 7 is a pretty solid language. I don't like WordPress at all, but there's so much cruft, legacy nonsense and plugin garbage that you won't win that battle.