The whole scenario is highly improbable. Our best conclusion is that even the basic conditions for this prophecy will fail to materialize. Therefore, we are justified in calling it a false prophecy. Uncertainty is unavoidable. The simple act of determining whether a prophecy has succeeded or failed necessarily invokes some uncertainty.
Absolute certainty is lost at many different levels, practical and theoretical.
Our conclusion about Ezekiel's prophecy of Egypt is none the worse because we have not waited until the end of time. The case for a successful prophecy, even given the unexpected nature of prophecy, has been ruled out as our best conclusion, and that's all that matters. However, we actually deal with probabilities in real life. An open-ended prophecy requiring a highly improbable future to even set the stage is no better than a completed prophecy whose failure has been judged highly probable. We could be wrong in either case, but if we have weighed both sides and found the prophecy highly improbable, then we are justified in calling it a false prophecy.