Ethnicity is a matter of self-identification.  Language is only relevant to the extent that it influences people's self-identification.  If people don't feel they're ethnically Iranian, but only Persian or Kurdish or Ossete or whatever, so that we have to rely on language, then there's really no such thing as Iranian people, and this article should be redirected to Iranian languages.  If "Iranian" is a real thing, then language isn't critical.  So: do Persians, Parsis, Tajiks, Kurds, and Ossetes think of themselves as being one people, or part of a coherent group of peoples?  If so, then "Iranian" is a real thing.  If not, it's an invention of linguists or historians.  —