real.geizterfahr: Microsoft stopped selling Windows 7 because we live in a fast developing world. It's easier to create a new OS than to patch your old OS for new standards and to deal with angry cutomers because you broke compatibility with something.
agogfan: I'm not really sure what Windows 10 offers me that I would choose it over Windows 7. However, I realise I can't speak for other users. Personally I've realised that it's better to have a consistent experience for longer rather than trying to have the latest and greatest and continually spending valuable time on learning to support it.
So looking at it from Microsoft's point of view, what benefit does Microsoft get from Windows 10 versus Windows 7? They're giving it away for free so we can rule out that they trying to force more sales in the short run. In the long run, we can expect them to try to make their operating system dominant over more types of devices in the future, thus expanding their market.
Hardware vendors will support whatever dominant operating systems there are, so if Windows 10 becomes dominant for good reason, then I can understand them wanting to drop supporting drivers for less popular operating systems.
So that leaves developers. I believe Windows 10 is supposed to offer a unified code base across PCs, tablets and smartphones, so perhaps that's a good reason to push Windows 10 as it will lower the development costs for programmers since their programs will support more devices and hence the cost of software programs should fall in general?
If Windows remains the dominent OS, they can make a lot money off the extras and spinoffs, by releasing Windows-specific software and tooling: Sell office for a hundred bucks, sell Visual Studio for a couple of hundred bucks to developers making software for Windows, sell Windows Server for a thousand bucks to admins that are hooked on Windows. Charge hardware manufacturers for that coveted "For Windows" sticker, etc.
I'm pretty sure that Microsoft has been making most of it's money from the Enterprise and its peripheral technologies for a very long time now and it's probably what pisses me the most about the Microsoft experience: Not the fact that you pay a, relatively minimal, fee for the regular version of the OS, but that you are locked in the Microsoft ecosystem which is both restrictive and a money drain if you aren't a fortune 500 company.
They've recently made strides to shake some of that heavy-handedness off (open-sourcing .Net, supporting Node.js and various open tools on their platforms, working on support for containers, etc) and I think it's great. However, they are doing it as a reaction to changing realities in a marketplace where Windows is only dominant on the desktop whose share of the market is rapidly shriking (servers, embedded device, smartphones, heck even supercomputers... they lose to Unix/Linux in every other market).
In comparison, the Linux community does the open ecosystem thing better, has been doing it for a lot longer and actually wants to do it: no coercision from the market is necessary.