Close, but no banana.
You're so tantalisingly close to the truth, yet haven't quite got it.
Your argument, after all, can also apply to future versions of Windows/Mac OS X. Should their creators ever be so stupid as to drastically change the interface, that is.
(Silence at the back! Wait 'till I've finished, as that's not my point!...)
The truth is that familiar interfaces are nice, but as you indicate with your tablet example, people are always willing to change if the benefits are clear.
What people *really* want is functionality.
After all, how many Mac users have a copy of Office? By many people's logic, they should. Office is irreplacable, and LibreOffice/OpenOffice just aren't good enough!
Yet I see more Macs with iWorks than Office on them...
Most PCs come with a trial version of Office these days. And most people don't want to pay 100 bucks for it at the end of the trial. Show them LibreOffice/OpenOffice with 100 of their bucks in one hand, and a copy of Office with a receipt for 100 of their bucks in the other, and they soon decide how valuable Office really is to them.
And yes, I have seen people pick Office instead of the 100 bucks. Usually for business reasons. But then, I also see small businesses just using Google Apps instead.
And Google Apps shows the real direction for functionality these days. Web browsers just won't be reasonable old chaps and stop their pace of development. They insist on rudely and uncouthly becoming more and more capable. It's just not cricket!
Remember Microsoft Works? Not the standalone abomination, but the Word/Works/Autoroute/Other bundle from the early 2000's? I knew people who loved that. Mostly for Word and Autoroute - that made their computer useful to them. These days a browser with bookmarks to Google Docs/Office 365 and Google Maps/AA's website/RAC's website could do the same. For free.
What software do people really want from their computer so that they can do what they want to? I'd venture the list looks a little like this:
1. A web browser.
1a. Email. (Maybe via browser, maybe via IMAP/POP3 See 1).
2. Basic Word processing.
3. Plays music. (Maybe via a web browser? See 1.)
4. Plays video. (Maybe via a web browser? See 1.)
5. Allows basic photo management/editing/sharing.
6. Using a spreadsheet as an overgrown table creator, and, in a very small minority of cases, using some basic formulas - but I wouldn't hold your breath waiting for that last bit to happen.
7. Abusing DTP to create godawful invitations to equally godawful events, and brain-melting newsletters about said godawful events - newsletters that are so bad your hamster will bite you for forcing it to **** on their shredded remains.
8. Porn (see 4).
I hesitated before putting 6 and 7 on. Many people will never actually use a spreadsheet, despite being aware it's there. Most home DTP functions are basically templated, and could probably be done in most word processor packages anyway.
The basics of everything on that list can be done in a browser if necessary. And other things I've probably overlooked.
So Microsoft's biggest fear is that customers - or on their behalf, OEMs - begin to ask why Office costs them 100 bucks when they use it so little. Or why Windows adds 20 to 30 bucks to the cost of a device, when all they're going to do is use a web browser that's free anyway.
At that point, Linux looks much more attractive to OEMs, and I'm sure many of them have been hinting at that since the netbook era.
None of this affects Apple as much. They're a luxury brand. It actually helps Apple, in a way - knowing that you don't need Office but just need to write simple documents occasionally makes a purchase of iWorks easier.
The tablet/phone markets are a little different, partly due to a richer platform experience due to well thought out APIs and partly due to a much lower cost per app, which distorts things a little.
But sticking to the fundamental "how do I do this?" rather than "how do I run XYZ?" shows much the same results - lots of movement towards web-based services. (A shocking number of apps seem to be little more than wrappers for mobile web sites!)
When you wrote "Linux is not an OS for most users, and will never be", you could have put Windows 8 in there. Or even Mac OS X. And you'd still be correct.
Because whilst a Linux distribution comes with plenty of software - free, curated, easily installable - all it needs to satisfy 90% of the needs of 90% of consumers is a web browser.
Most users don't need an OS, except as a bootstrap to a web browser. To them, Windows is what DOS was for the Windows 95 developers - a handy way to start the ball rolling after the power button is pressed. A minor step on the way to the final destination.
Sure, some older folks are stuck in their ways. (And many more aren't.) But by the time today's kids can afford to buy a copy of Windows or Office with their own wage packet, they'll already have gone through multiple versions or alternatives, and learned that they don't need it if they can run a web browser for free.
Welcome to the future. Feel free to bookmark it in case you need to visit again...
(Who am I kidding? Just type "the future" in your address bar, and it'll pop up! Remember using bookmarks instead of Google and local browser history? How quaint!)