It might initially sound odd, but I think MS's biggest problem was when they didn't do a bad job of making an OS.
Prior to Win10, MS's business model was based on people buying new versions of software. As a result, when MS made a good version of Windows it threatened their revenue over time due to people not buying new versions of Windows. For example, my usage of Windows has been 3.1, 95, 98SE, XP, 7. I only upgraded the last 2 times when security support ended because the newer versions of Windows didn't give me enough extra to justify the cost. An OS doesn't wear out or go off, so why buy a new one when the one you already have works fine?
10 is different because MS has changed the basis of their OS business from making an OS to making spyware and a rootkit that has an OS attached to it. That's why they are so determined to steamroller everyone into having 10 - with 10, it's not the OS that's the product. It's the users that are the product. It's a shame, because the OS itself genuinely is an improvement over 7. Enough of an improvement for me to have bought it if it was just the OS. The interface is different and in many ways inferior, but there are ways to modify it to make it as good as the interface in 7. But the spyware, rootkit and MS's dogged determination to control user's PC's through it come what may mean that 10 is inherently insecure and unreliable. If you use 10, you can never, ever, be sure that it will carry on working because MS can and do break it remotely and repeatedly. You can be sure that you have neither security nor privacy, though.
I can understand why they've done it and I've no doubt it will make them money because most people will stick with Windows regardless and put up with the insecurity, unreliability and lack of privacy. But I don't like it and I don't want it and I know enough and have put enough time in to be able to evade MS's attempts to force it on me. I'm still using 7 and will do so until MS make it impossible to do so or I switch to a better OS.
My workplace still uses XP, which I think is taking "it still works just fine" too far. It's not even supported with security patches any more.
The only thing I'd use 10 for (or recommend anyone else use 10 for) is a PC that's used solely as a games console and then only because of DX12 (which MS won't port to 7 or 8 because it's currently the only good reason to use 10 by choice).
I'll probably switch to 10 for my games console PC and build a very small very quiet basic PC running Ubuntu for everything else. Or maybe use one of my 3 spare PCs for it, but I'm kind of in the mood to see how small and quiet I can go for it. I considered buying a PS4, but they're just not as good for gaming. I could get used to the adequate graphics and the almost adequate framerate, but I couldn't get used to the uncomfortable and inaccurate controller.