If it's been JITed then the AST may well have been discarded after the JIT completed! Doesn't V8 work like that today? You'll need to pass it again even if you have already parsed it!

It's not hard to think of worst-cases. Think about an enormous function that you would not want to inline. It's been JITed, or it's running in a byte code interpreter, and the AST has been discarded. In your scheme you'll have to parse it to find it's too big. Huge waste of time, if you can see it's too large from the source code.