Well the most probable reason for the MIRs and no real price drop is that the 400 series cards are still in high demand for mining due to their mainly lower price so it's to be expected. Also like expected, 500 series isn't a huge boost, Vega still isn't out yet and that will be the big show coming up.
Production of all 470/480 cards stopped a few weeks ago, AMD not selling anymore parts, whatever is left in the channel will be sold but there are not anymore coming unless AMD changes course and releases more parts...
The only parts that AMD sold were the Polaris GPU which is exactly the same as the 570/580.