I am one of these users. Linux is not challenging or for hobbyists. That's the old Linux and does not apply anymore. It's simply different and once you learn your way around it can be as easy as any Windows version if not easier. Even for a "normal" Windows user who only wants to do mundane things with the OS like browsing the web, using email Linux can be more appealing than Windows. Here is one of these normal Windows users not a Linux power user who choose Linux over Windows in an experiment conducted by her son. This is her trying Linux - https://www.youtube.com/watch?v=I-PTQVeCF9k This is her trying Windows 8 - https://www.youtube.com/watch?v=1ujmDrcKWo8These people anyway are more knowledgeable of computers in general and aren't likely to allow a "challenging OS" to put them off their purchases. Many of them are also hobbyists and probably relish the challenge in any case and a lot design and custom build their own rigs.
Did you know Surface tablets with Windows 8 RT are locked down so you cant dual boot with them at all? Did you know all PC's that come preinstalled with Windows 8 on non ARM systems has UEFI and Secure Boot enabled? Even if you disable Secure Boot and have UEFI compatibility built into to your non Microsoft operating system, UEFI is so buggy that most of the time you still cannot dual boot properly. No matter how savvy a Linux user is, they cannot do anything about this until UEFI itself grows up and the firmware industry gets the bugs out. I really think your underestimating the numbers of dual boot users out there who are now shunning preinstalled Windows 8 machines due to these issues.
I realize this and in fact agreed with you that this does also play a part in the decline of PC sales. Why lug a laptop around when you can do almost everything you want to do with a smart phone? Although for many people like me, I would rather the PC laptop for more power and flexibility ( I use it mostly for games I can't play on a smart phone) . I don't own a smart phone because paying for an extra service and having a scaled down laptop I don't need. It's a waste of money for me - and I can still make PC to phone phone calls from my laptop if i need to.What some people don't seem to realize is that the ENTIRE market for PC's as we know them (and for similar products from our favourite "Fruit company too") is for all sorts of reasons --NOT RELATED TO THE OS is declining.
Even people like me can go for days at a time not even LOOKING at a PC -- a lot of non work stuff I need to do doesn't require the use of a full blown PC anymore -- I can even copy and Print a lot of stuff without even having the computer switched on -- just print directly to a network printer from your phone for example.