But Microsoft is good about not being realistic with some of their approaches. They saw the popularity of the iPod and iTunes and created the Zune, now defunct. They see Apple's app store and they want that money, too. Giving away Windows 10 for free allows them to set that foundation. Because not only is it free - as this very article states, they're actively trying to push it on everyone. That's rather novel behavior for them. We have to fight to not install this update. Apple's made OS X updates for free, but never with this kind of forced-upgrade pressure.
Microsoft's motivations always come down to money via market dominance. Given that Windows and Office are (generally speaking) their prime sources of income, when they decide to give away a full update of Windows for free... then there's another way they intend to recover that money. The app store, sparse though it may be right now, is the most reasonable conclusion I've been able to draw. After all, if their app store suddenly fills up with apps but the majority of their users are on Windows 7, that's a lot of lost sales.
Get the App store on everyone's system first, and that's a major obstacle overcome. Unlike the present vicious-circle failure of trying to push a new mobile ecosystem, there's a huge and entrenched Windows market already. Now they can promise app developers all of these extra eyeballs via Windows 10.