The point is that neither the historical story of the battle at Thermopylae nor the ficitonal adaptation of it in '300' is about the "age-old struggle between good and evil". There aren't any good guys in this story, just guys who maybe are a smidge less ruthless and bent on forging an empire.
Even today, our civilization hangs by an invisible thread, that links us to the future. I am so glad that the Spartans "lost" at Thermopylae, so that the Athenians, Spartans, Corinthians, Thebans, etc. could win at Salamis and Plataea, and for that matter... that Franks won at Tours, that the Venetians won at Lepanto, and that the Austrians turned back the Turks at the gates of the West. Thank you, Leonidas and brother Spartan warriors for your sacrifice.