There are no pure peoples on Earth. [Edit, for the most part]. Conquerors in general intermarry with conquered people. Even in cases of massive genocide, there will be some genetic contribution to the conquerors. Massive genocide happens, but is not as common as linguistic and cultural replacement.

As proof, look at the small, but definite Neanderthal and Denisovan traces in non-Africans, and in Africa the traces of some other species of Homo, perhaps Erectus, that DNA shows. Archaic humans, very different from our species, still mated with Homo Sapiens. The difference between a Neanderthal in the Balkans and some recent immigrant who still looked somewhat African would have been an order of magnitude bigger than the difference between, say, a Tibetan native and a Zulu woman. But people still got together and had sex; they exchanged ideas; attitudes; religion, etc.

In so doing, it becomes clear that modern humans are not the same as the original modern humans, but transformed somewhat, however slightly.

The Aryans seem to have originated in the plains region just north of the Caucasus; and to have migrated East toward the Tarim region, West into Europe, and to the S/E into Iran and, finally, India. As they did so, they came into contact, in peace and war, with other peoples. A similar process happened. Some of these earlier inhabitants would be marginalized to extinction, some would join the newcomers; some would leave; some would get enslaved; some would merge easily into the new order.

The presence of previous tenants in Iran is proven by the Elamite civilization, which seems to have not spoken an Indo European language. The Elamites were eventually absorbed by Iranian peoples and Semitic ones.
And Alexander's men encountered the Icthyophagi along the Indian Ocean coast, who were living a Neolithic, beach-combing lifestyle. These people were not Aryans either, as if they were, they would have been Iranian agriculturalists of some sort. Instead, as author Robin Lane Fox points out in his work The Search For Alexander, they had no iron tools, lived in huts made of whalebone, and were in poverty. Like the Elamites, the Icthyophagi are no longer a separate people.

Those who conquered into Iran also affected the cultural mix, but even if the Mongols, Turks, Arabs, and Macedonians had never come, they still would be a mixed people in fact. They picked up influences from those they conquered or otherwise integrated with. In the last several thousand years, when some human group expands, it will do so into a region already occupied by another group, and will expand due to some competitive advantage over the old timers. An exception (as in the case of Polynesians) is expansion into some region that was uninhabited.

We cannot consider the Iranians as the original Aryans. Rather, we have successor peoples, all of whom are as different from one another as they are different from their founders, but have a large Aryan heritage, especially in language.