If the Grand Duchy of Armenia is anything like how Russia treated Poland, I think Armenians would rather live in Greece. In Russian Poland, Russification policies meant the language could not even be taught. Way worse deal than what the Armenians would be getting under the Greeks.
Here's the population exchange proposal: roughly 3+ million Greeks/Armenians/Assyrians from Turkey and roughly 3+ million Turks from European Turkey + Western Anatolia. That leaves the Turks with a large population in western Anatolia but perhaps less than 50%. Historically only about 300,000 Turks left "European Turkey", so here there will be 10x as many fleeing Western Anatolia.
It IS ethnic cleansing, but on both sides. Greeks and Armenians will be forced to leave Turkish areas of Anatolia as well. Of course this is implying a scenario where "Turkey" is utterly defeated by Greece in a major war with significant European powers involvement on both sides. The fact that the Turks will try to massacre Greeks + Christian minorities in large numbers should be sufficient for whichever European power is supporting Greece to agree to punish them. OTL they didn't want to devote any resources to punishing the powerful Turkish state. But with a far stronger Greece TTL, Turkey stands to lose a LOT of land, react badly and then keep losing more land as a result.
If Greece only holds onto coastal areas, the situation will be even more unstable with Greco-Turkish wars going on forever + genocides. You either defeat the Turks so badly, they no longer pose a threat (in a major war). Or they will keep conspiring to retake all of Anatolia and making alliances with major powers to do so. Greece will be at war with Turkey for many generations. In this scenario, of course, Greco-Turkish tensions in western Anatolia would be tense and hostile potentially for decades. But still a happier scenario than OTL genocide of Greeks, Armenians and Assyrians and endless wars with a powerful Turkey in my view.
Here is an OTL pre-1910 map of ethnic populations and consider whether it is possible for Greece to take most of Western Anatolia.
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