Actually, the Ottoman Empire for most of its existence was near to the ideal of a multicultural society. Each ethnic/religious group was classed as a millet, with its own laws and leaders. The Sultan ruled over all the millets, but as long as they obeyed him they were generally left alone to run their own affairs.
Most of the massacres of Greeks, Armenians, etc. you reference took off in the 19th and early 20th centuries, when the Turks were trying to become a "real" nation-state of the European type. Young Turks and all that.
As with European nation-states, "minorities" had to be assimilated, expelled or exterminated if Turkey was to become a real nation.
I don't mean to imply the Ottoman Empire was some sort of tolerant paradise in early days, but there is no question that the Turks trying to apply European-style nationalism made things worse for minorities.
That the minorities had their own nationalisms, in many cases claiming the same land, certainly didn't help.
"Millet" defined as: the separate legal courts pertaining to "personal law" under which communities (Muslim Sharia, Christian Canon law and Jewish Halakha law abiding) were allowed to rule themselves under their own system.
Then one can argue that Iran today, under the Mullahs' rule, has been running a near to the ideal of a multicultural society, a type of "millet", as well.
So long as other officially recognized 'religious' communities obey the Supreme Leader (a type of Sultan or Caliph), respect the fact that Iran is officially a Shia Islamic State, and publicly adhere to wider Islamic laws & rule (e.g. dress code/hejab), then they are free to run their own affairs within their own community (but, not when those affairs involve a moslem). That right is legally protected in the Islamic Republic Constitution. Religious minorities are even permitted to elect a representative to represent them in the Parliament, in Iran.
Actually, the concept of "millet" was used for the communities of the Church of the East under the Zoroastrian Sassanid Persia in the 4th century [long] before establishment of the Ottoman Empire. Related posts
The difference is that the Sassanid of Persia were not pressuring others to convert to Zoroastrianism, by direct or indirect means. Nor did they mind if a Zoroastrian by birth converted to another religion, as long as the convert stayed loyal to the Persian Empire. This, to me, is closer to 'nationalism' than millet associated with Islam or any Islamic Empire/Dynasty.