People don't have to buy a lot of applications, but applications bought could have different interfaces and behavior when used on laptops vs tablets/phones. Using Skype? The laptop version could include the IM section, but that could be excluded from the tablet/phone interface. Just as an example.
On the phone side, one thing Microsoft has gotten right is the Universal app store. That way, I can run Yelp! - currently available only on the phone OSs - on a tablet or even laptop, w/o opening a browser. Hope it also solves one issue Windows Phone 8.x had - most apps being 'web containers'
Your point is spot on. Apple wouldn't have cared and Linux would have done it both ways until people got used to the change. Microsoft's Windows 8 was supposed to be a transitional operating system with touch becoming an important and key part. All applications needed to have a reasonable touch mode. They ultimately have pulled back from touch which means they pulled back from ubiquitous computing ( https://www.youtube.com/watch?... ) Windows 8 was there last chance to prevent themselves from being a niche: the standard enterprise desktop product. Now that's still a huge niche but moving from the default everything to just one particular submarket is a big loss.
But non touch laptops are completely different things from tablets, phones and touch laptops. It never made sense having the same interface for both. It's trying to wear hand mitts on your feet instead of socks, or socks on your hands instead of gloves.
If you recall, previous versions of Windows CE and Windows Mobile had the same UI as Windows XP. Which was a bad interface for those things. In Windows 8, they flipped the mistake, not forcing a touch interface for desktops & laptops and having that long horizontal scrollbar if you don't have a touch screen laptop (which I in any case don't want to touch & smudge)
The other thing is that while the Metro interface was actually good for Metro applications, if you open one in Windows 8.1, it goes into the desktop and then opens it. Why? A lot of the apps that Microsoft bundles, like News, Food, Health, Games et al can be opened w/o needing the desktop at all. The desktop should only be there if an app is invoked from it. Such as Notepad, or Office, et al.
In Windows 10, while the restoration of the 7 like interface is somewhat good (except for forcing the large icons to show up when you press the Start button), the tablet mode is worse. What it does is just rearrange the screen, putting the icons on the right and the entire list of apps on the left. They could have just left the Metro interface there intact. I have a Winbook on which I had been testing the Preview Edition of 10 (will download the RTM this Sunday) and Tablet mode has been tough to deal w/.