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Dedieu is a by-the-numbers kind of analyst, who whips out the charts early and isn't afraid to extrapolate.
For Microsoft to maintain their profitability, they have to find a way of obtaining $80 of profit per device. Under the current structure, device makers will not pay $55 per Windows license per device and users will not spend $68 per Office bundle per tablet. Price competition with Android tablets which have no software licensing costs and with iPad which has very cheap software means that a $300 tablet with a $68 software bill will not be competitive or profitable.
However, if Microsoft can sell a $400 (on average) device bundled with its software, and is able to get 20% margins then Microsoft is back to its $80 profit per device sold. This, I believe, is a large part of the practical motivation behind the Surface product.
Didieu is a master at analyzing Apple's moves. It's less clear that his math is as well grounded for Microsoft's very different business.