I say both, one could argue. Honestly, Microsoft already has a pretty good OS with Windows 7 (pretty good, not great) that has sold in record numbers in such a short time span that will be supported until 2020-2022 about there; on the desktop form factor. That's the big thing there, because Windows 7 as much as it was designed for touch (notice the large window control buttons, the larger Taskbar, and a few other touch UI improvements over vista) didn't take hold really at all on the current generation of tablet PCs. It's kind of like how Bill Gates said that Windows Mobile wasn't taken to certain places before ios did, such as an app store or multimedia marketplace among other things. Windows 7 doesn't cut it for the tablet or even touch form factor that well. Windows 7 is already solidified pretty well on the desktop and laptop.
Now enter Windows 8, designed for "new" PCs, new PCs meaning tablet PCs since the Windows tablet has been around for a good decade but never came to being known until 8. Windows 8 has the things needed for a proper touch and tablet experience while at the same time keeping the things Windows 7 improved upon. This one here is the gamble as it's kindling a new PC form factor of the tablet as well as propelling touch on other traditional form factors. Microsoft has the will to do that and the ability to do so rather safely as people who really H8 Windows 8 can just buy Windows 7, no loss to them basically. People that love Windows 8 stay on 8. If they have it installed on an existing PC, that's a gain to them but loss to the modern PC market. If someone buys a new Windows 8 tablet PC, they may give up their old laptop or desktop for the tablet. Or they may even buy Windows 8 to install over on their existing PC, but prefer the touch aspect of the tablet.
Windows 8 is also essentially wrapping the old into the new. Windows 7 didn't work as a tablet OS because it is an old UI paradigm, a start menu doesn't work very nicely with a finger. Windows 8 took the Desktop and changed the start menu to a Start Screen, old evolved to new. That's the main point with 8, it's taking the existing Windows of the '90s to Windows of the post-early 21st century.
So it's a gamble, it's bold, possibly arrogant, but there is a strategy regardless of analyst's PC apocalypse predictions (some are really idiotic as a PC isn't regarded as a desktop anymore, it can be a laptop, tablet or desktop, something certain analysts don't understand) about Microsoft. If it'll pay off is yet to be seen, as it takes a few years before a new UI/OS is proven. Windows Phone for example here, some naysayers cry about how the metro UI of it isn't attractive to consumers. Yet, in less than six months, Windows Phone 8 got to a marketshare that took Windows Phone 7 about a year or so more. It has doubled in usage in the US, and is outselling the iphone in the top six emerging countries that shouldn't be overlooked. It is the most used platform in China, an up and coming economy that would be dreadful to miss out on. Time is what it takes.
