First of all Microsoft's windows 10 is a huge improvement of win8 and it has lots of familiar features, introduced earlier in win7. It is also easy to use, so those people who cannot take advantage of the free upgrade or are buying a new pc will pay for windows. That will certainly save Microsoft, though I don't think Microsoft is in such danger.
On one hand, Microsoft's reputation seems to be at an all-time low. Clearly tech-nerds have lambasted the company, their operating system, and especially their web browser, Internet Explorer, for at least a decade now. But their mainstay has always been that enterprise values the ease of use of Microsoft products. They upset that customer base with the odd interface of Windows 8, but Windows 10 will fix all of that, and maybe more.
After the slow uptake of the Windows 8 operating system, it's evident to see that Microsoft wanted to try a more pioneering strategy. Even if the uptake of Windows 10 is slow at first, it will invigorate software developers who will want to write code for a burgeoning and groundbreaking operating system.
Microsoft is only going to make money off of it because they make so much off of advertising, and the only people who will be fascinated by the design of Windows 10 are the high-end tech geeks and those gullible enough to think that Windows is "cooler than Apple." .
Even though Apple has soared since the turn of the century, Microsoft is not in any danger. Windows 10 will not save Microsoft because Microsoft is not in need of saving. That said, there have been diminishing returns on Microsoft's operating systems since Windows 7, still the best format for operating Windows computers, and many will approach 10 with skepticism, but Microsoft is working hard on their Windows Phone line and starting to garner a vocal userbase, and the Xbox, despite its faults, will continue to be a success in spite of the controversy.
I am not a huge tech person, however I have heard absolutely no chatter that Microsoft is in trouble. So to me I would say that the new Windows system won't have to save Microsoft. It may return it to the top but I don't see a scenario where Microsoft is in mortal danger.