No. Because they want to sell much and often.

Nah. Amazon doesn't really care what you buy, as long as you buy something (from them). See also: Netflix

No, because their incentive goes the other way around: they want users to buy lots of things.

Reviews will eventually have to rely on non-store, external sites.

This doesn't make sense to me as it has the opposite effect. If I want to buy an item, I'm pretty dead set on buying it. Now it's just to figure out which company's to buy. If I can't rely on reviews and get a terrible unit, I will return it and no longer trust Amazon for that item and turn elsewhere. Ultimately, Amazon lost some money, I lost some time, and Amazon lost the sale.