Firstly, the modern "Arab" will only have a very small fraction of his ancestry originating in Arabia. The genetic makeup of the region is largely as it has been since prehistoric times; do you seriously think that a collection of nomadic desert tribes could significantly impact, let alone supplant, the population of the fertile crescent, one of the most populous regions on the planet for the entirety of the pre-modern era. The Arab expansions did not represent a major population shift such as the European migration period, and, even if it did, so what? The peoples living in the pre-Arabic Middle East would've looked much the same as the Arabs did, possibly with lighter skin tones. And, as the fertile crescent was the most populous region of the Persian Empire and the source of the bulk of their armies, their troops would reflect that.
What's more, much of the same applies to the Persians. The Aryan expansions, while representing a greater shift in the genetic makeup of a region, were still relatively minor, and there is little evidence to suggest that the "Aryan" Iranians would've looked radically different from other Middle Easterners anyway- they don't today, after all. Don't confuse "Aryan" in the Indo-Iranian sense with the Nordicist use of the term; the two have
little in common.
Typically, historical population shifts do not represent a major change in the genetic origins of a region. Ethnicity may alter dramatically, but this is as much through self-identification as anything else. The invading group typically sets itself up as a ruling class, while the peasantry remains much the same as ever. Even in England, where the Germanic migrations had a greater effect than on anywhere else in Europe, the genetic makeup of the population is rarely more than 50% Germanic, and much of the remaining pre-Germanic makeup is pre-Celtic, dating back to the Neolithic. The European domination of the New World is an
exception to an historical pattern, it is in no way an effective comparison for any pre-modern human migration. It represents an entirely unique set of circumstances that bear no comparison to those which occurred during either the Aryan or Arabic expansions.
And the
Aryan expansions did indeed pre-date the Persian Empire, as evidenced by historically recorded pre-Persian Indo-Iranian nations such as the
Median Empire (to which the Persian Empire was a successor state).
Well, that's an over-simplification, but, either way, it's
irrelevant. As I have said several times now, the actual systems in place do not matter, what matters is how the Spartans
regarded those systems. You can shout "Sparta was wrong" until your blue in the face, that doesn't change the fact that the Spartans regarded themselves as free citizens and the Persians as the slaves of an autocrat.
That was what the film was attempting to represent and how
you feel about the political situation at the time is irrelevant.