First, you don't have to buy a new computer to get a newer version of Windows (especially since 10 seems to run just as well as 7/8 on the same hardware, if not better). Second, since MS is apparently offering free upgrades to 10 for consumers on 7/8 it doesn't seem like cost is really a factor. The China case is interesting, but I'd like to see some more recent figures on usage, since that article is about 2 years old. I did some quick searching, but everything I could find seemed to be from around that timeframe.
I don't know that the browser is a primary carrot, but it's part of the deal. If you can get enough of their new software to run on, say, Windows 7 to duplicate major functionality of 10, what's the point in upgrading at all? It doesn't make sense to have (people who will stick with the old OS no matter what) + (people who might upgrade, but don't because they can keep the old one and still get the cool new features of the new one) using older versions of Windows, thus increasing maintenance on old software.
> 1/2: I'd posit the people using XP in 2015 are probably not the ones capable of upgrading their existing OS. New OS means new computer for them.
Are those people capable of installing a new browser after making the informed decision that it's better than IE 8?
> 3: Users of every other OS are enticed by features other than the browser. Only Windows ties the OS to the browser.
Every other OS? Let's look at the major players in desktop computing: Windows and OSX. Windows has IE/Spartan, you can as of now only get them on Windows. OSX has Safari, there was an abortive attempt to develop it for Windows but it didn't take off and is now abandoned. I don't think this is too surprising. Here I am on OSX Mavericks making this post from Safari 7.1.3 because Safari 8 is only available for Yosemite. Given that the browser is the main piece of software that many consumers interact with, I think it's fair to say that the browser is still a significant value proposition, even if it's not the only one.
I don't know that the browser is a primary carrot, but it's part of the deal. If you can get enough of their new software to run on, say, Windows 7 to duplicate major functionality of 10, what's the point in upgrading at all? It doesn't make sense to have (people who will stick with the old OS no matter what) + (people who might upgrade, but don't because they can keep the old one and still get the cool new features of the new one) using older versions of Windows, thus increasing maintenance on old software.