In retrospect, it seems that the Greeks were doomed from the start. The Battle of Marathon was a battle they could not win against huge numbers of Persian soldiers. It was a battle they had to win to retain their existence as a culture and they lost. After that, the empire began the process of dehellenization.
Even more important though, was the colonization first of Greece, and then of Illyria, Scythia and finally the whole of Europe save the lands of the Celts and those of the Finns. It was not until the coming of the Prophet (pbuh) that the empire was swept away.
Imagine how it might have been in the unlikely event of a Greek victory. The Persian army would have been humiliated and driven back in disgrace. Perhaps the whole of Europe may have become Hellenized, all worshipping the Gods of Olympus (or perhaps all Christian or Moslem after the coming of the Prophet.
Sometimes, when we have no hope at all, we must still stand firm in the face of our enemies. BUt alas, the Greeks had not learned that lesson at Marathon.