Fair enough; that's true when it comes to an arbitrary program. However, consider a program that contains both heaven and hell in different branches, and will take one of these different branches depending on the interpreter. Or, alternatively, consider a program simulating a "good" world that will, given some small tweak in the orignal interpreter, simulate a much worse world because some simple but essential thing will be off. Such thought experiments, as far as I see, override this objection.