Wii did over 12 million in it's peak year in the same Sept-Dec timeframe, so 11 million with a global $100 price cut is definitely possible. PS3 did 8.8 million in 2008 for that period. PS4 did 8.6 million in 2014 at $399. PS4 should do much better than that with the same price cut and the same price point as PS3 in 2008.
But I'm not sure where 9+ million in sales is coming from between March and September. 1.5 million per month on average over the doldrum months seems very unlikely. More than likely PS4 will be flat or marginally down on 2014 at the current price, and the doldrum months last year never went above 750K. And the OP thinks PS4 will double that this year for no reason other than a few games? Batman could help June get to 1 million, but that's about it.
So with that in mind I think 25 million by 1 Sept is optimistic + 11 million with a $100 price cut = 36 million end of 2015.
$50 price cut and awesome special deals for BF and in December I can see PS4 doing about 9-10 million so 34-35 million end of 2015 in that case.
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