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Personally, i want that. I've always been a fan of AOSP specially PA. But come on. Realistically, this will not happen ever. Just look at their business model. The reason they can keep selling phones at a very low price is because they rely on their services' revenue (MIUI) rather than the hardware revenue.
So if you really want this to happen, Xiaomi would have to compromise their business model by selling their phones at a higher price range to compensate for their loss.